You don't need to spend a lot of money on this - what you need is a programmer to hook you up to a simple shopping cart interfacing with phpbb so that you can sell poster related items, lamps and all the other related items that you could legally retail.
To put a value on the site alone would need some more information being given up. Is the number of uniques static or growing? In other words is the trend on an upward curve.
Search engine placements for major keywords (where are the sign ups coming from)? What application is the site written in (open source or custom programmed) and is is a mysql database etc. A lot of the info can be easily gleaned and if you are still using onestat.com as Alexa.com says you are then that would be a big bonus.
My view would be that until the site can prove that it can make money then it's not very valuable. So to value it on it's membership base alone I'd say you are looking at low £x,xxx for the site.
There is always the possibility that you have a membership base that you will always struggle to get them to part with their cash and if that's the case then the numbers may be impressive but without real meaningful revenue it's not worth a lot really.
On the flip side, if you can develop even a small amount of revenue from the existing membership base (say £500 per month) then you have a site that is worth 50k plus. I'm basing that on 3x yearly revenue plus potential for development.
Look in the back of private eye mag and see if you can team up with some of the retailers who proliferate the ad's pages and I'm sure you could sell advertising space to the tune of £1,000 per month in no time at all.