If you collect milsurps (as opposed to just buying them because they're cheap, or for plinkers, etc., etc.) it's of no relevance that a bayonet is something of an anachronism in modern times. In most cases the milsurps themselves are anachronisms, which is why they're surplus to begin with.
Yes, they can be expensive. Some cost nearly as much as the rifle to get a correct one in good shape. Just try finding a good-quality non-reproduction bayonet for a Japanese Arisaka or German Mauser. When I bought mine, those ranged between $100-$150, and prices continue to climb. Again, though, to a collector that may just be part of the process, irrespective of the bayonet's practicality.
For me, I collect WW2 milsurps. If the rifles I have would have been fitted with a bayonet during WW2, then I try to find one to go with the rifle. That, however, doesn't mean that I have bayonets for all my WW2 rifles. My M1 Carbine, for example, would not have had a bayonet lug during WW2, even though it was given one when it was rearsenaled after WW2. Hence, I have no M1 Carbine bayonet. For my Finn M39, it did have a lug during WW2, but history tells us that only a handful of Civil Guard units actually received bayonets. Since my rifle was not used by the Civil Guard, I'm not searching for a bayonet for it.