Some are and some aren't expensive - bayonets that is. 1907s have been creeping up in price ever since 2014 and the Centennial of the Great War.
No.4 bayonets are interesting. The No.4 series of bayonets started with the No.4 Mk.I - a milled cruciform spike bayonet, very pricey $200-300 is common as total production was only 75,000 made by Singer of Scotland (the sewing machine company). The Mk.II and Mk.II* were made in the millions, are stupidly common and I wouldn't pay more than $30 for an excellent condition example. Having said that, a Canadian Arsenals No.4 Mk.II can command up to $50, because only 5,000 were made during the Korean War. The final, and crudest variant is the No.4 Mk.III, it's hideous and frankly I think everyone was embarrassed that this was even approved because only ~192,000 were made. The No.4 series of bayonets was universally hated by troops, unlike the 1907, and was made obsolete in 1946. Of course, with a No.4 rifle you also open yourself up to getting a No.7 bayonet (pricey, in the $300 or so range) or a No.9 bayonet (bit more expensive than a No.4 bayonet)...
The M1917 and the British Pattern 1914 bayonets will only mount to the M1917 rifle and the P14 - and are interchangeable between the two rifles. The two notches in the grip scales were made to easily differentiate the bayonets from the 1907 Pattern as the bayonets are nearly identical; other than the very different sizes of muzzle rings.