Senior financial experts have cast doubts on the government's decision to use a 30% discount on the €77bn loans that Nama will acquire from the Irish banks for €54bn, saying it did not take into account liquidity factors in particular.

They said that three of the six non-Irish banks active here are willing to take a further discount on their loans to exit the market which should have lowered further the price for the assets of banks that are covered by Nama.

"Also, as assets are being transferred to Nama at a 10% premium, it implies buyers would be paying 10% over the odds. There's still no financing available to finance exits which implies another discount for the lack of liquidity… My view is the Nama price is another 20% over today's values," said one industry expert who asked not to be named.

Additionally, we learned last week that the average loan-to-value rate for the loans is about 77%, 10% more than finance minister Brian Lenihan had suggested, confirming the Sunday Tribune's consistent assertions that personal guarantees and cross-collaterised loans were often used instead of deposits. Based on the Department of Finance figures last week, there should have been another €7bn of hard cash used as deposits and its absence thereby exacerbated the current crisis.

Almost 2,000 customers with about 21,500 loans have been identified for potential transfer to Nama and of the €77bn, €9bn is accounted for by interest roll-up. Senior property industry sources suggested a non-embarrassment clause should be insisted upon by Nama when it comes to selling on portfolios of loans or properties. That would allow them share in any upswing if the assets are sold on quickly because it gives additional financial compensation if a successful purchaser flips on the collateral within a certain period of time for a higher price. At least one semi-state insisted on such a clause when it sold off property in recent years.

Market sources, however, poured cold water on the idea. "It's not a runner. There's still huge risk in residual bank portfolios under intensive care watch at AIB, Anglo, etc so I don't believe any private equity or financial buyer of Irish bank portfolios would agree to anti-embarrassment clauses," a market source said.

Concern was also ex­pressed about the yields being used to calculate values. The average prime yield in Dublin is 7.25% at present according to the figures supplied by the department, with prime yields on retail standing at 6.4%. But the deal by German fund Deka for the Tommy Hilfiger store on Grafton Street was done for in excess of this figure and it is one of the 10 best shops on what is regarded as the most prime retail street in Ireland. By contrast bids for Liffey Valley, one of the best shopping centres in the country, came in at a yield of more than 8.5% – European real estate investment firm Orion Capital was apparently the top bidder – even with an adjoining development site being thrown in for free. This highlights the real need for the valuation model to work correctly; done badly and we could be substantially overpaying for assets from the very beginning.

There is also a real need to take into effect the possibility of future rental decreases. While upward only rent legislation now looks unlikely to be abolished, the reality is that landlords are so desperate for tenants that many of them have agreed to lower rents rather than risk the occupants going bust. That is key in property because even in today's climate every €1 reduction in rent knocks about €14 off the value of your property in simplistic terms.

Lenihan said last week that provisions for falls in rents have been made in the valuation model being used. According to supplementary documentation from the Department of Finance, for the aggregate loan price "it is necessary for this exercise to take a view on the decline on the underlying property collateral since the loan was originated... Various factors, particularly geographic location and future cash flows including rental income will influence this valuation". That provides some comfort.

The banks have begun using Irish property advisers to value the properties they lent against and are insisting that a detailed process has to be followed. However, the problem with this is the yields being used could by definition be overly optimistic because they are based more on vendor expectations – meaning the banks in this case – than those of possible purchasers, meaning anybody other than Nama, which is largely the only game in town. "That's a fair point," said the director of one firm involved.

Another director angrily dismissed the valuation process being used, particularly in relation to land. He pointed to a residential zoned site in the south east which is currently being sold for half the price it looked set to achieve just three months ago and at less than 17% of its value at the peak of the market.

"The banks need [a certain] amount of money… and this is the way of giving it to them. My fear is that it'll be like dealing with the semi-states afterwards, they'll have a valuation on file and if you don't match that they won't entertain anything else," he said, adding that if an attitude like that persisted it would only change with time and, in the interim, "the market will stall for up to seven years until the political reality that we're never going to recoup this money sets in".

Others said their fear is that the loans on assets in Britain and elsewhere would be sold off too early in an effort to prove Nama was profitable, but leaving the state stuck with the worst of the loans and the true extent of the losses not being revealed for many years.