Hi mate, with a merlin xlm normally you can find a number on the rear derailleur hanger...for example 9647...which would mean it was build in 1996...the 4 could mean the day, and the 7 is the month it was made.
you may have to take the rear wheel off to see the number.
as for approximate value, you can search on ebay for similar item. I suppose this would be the fairest market value, however with anything that is not commonly sought but highly desired it would be best if you have a reserve price.
If I was selling that frame, and it is in excellent condition then I would not expect less than around 750 AUD$
If you live in the us then you have to convert the above amount to USD.
take care mate.