No idea. The 1917 marked ones seem to go for anywhere between $125 and $175, the Vietnam era seem to go for more than that. I haven't seen a 1918 marked one for sale.
As for rifle guys driving up the prices, probably to a small degree but I don't see them driving the prices up this high. Few rifle guys are interested in paying more for the bayonet than they paid for the rifle. Add into that the various markings and condition factors and I'm willing to bet that it is serious hardcore bayonet collectors that are driving up these specific prices. They were bringing about $100 16 months ago, then hit $160 about a year ago and now we seem to be in the $250-$375 range.
Case in point the 1917 bayonets. As a casual collector, I see no reason at all why a Vietnam era bayonet is bringing more than the original WWI issue. They are not as well made, they have the plastic grips, etc. Numbers may be fewer, I don't know but I expect it is guys who served in Vietnam that remember them that are driving up prices on that specific model.