My general policy is not to go over cover price, although I broke that rule probably only once when I purchased the Silver Surfer Omnibus at about 25% over. I managed to get a ton of OOP stuff by being patient and monitoring auction sites and the amazon marketplace. That was over the last 5-6 years. It is now understandably harder as time goes by. However, on the upside, Marvel has been reprinting certain OOP Omnibuses. Wolvie is a toughie. He's very popular but does Marvel want to reprint such an "old" Omnibus and carry on the chronological route in a Vol. 2 which it adopted in Vol. 1? The Epics are slightly different as they seem to follow a publishing order.
It really depends on you and how big a fan you are and whether that money could go towards something else.