Are you chasing YZ's or MX's? YZs are as rare as hens teeth and also as expensive. In Australia (different in the US believe it or not) There was the 73 YZA and then the 74YZ B - 125s, 250s and 360s (not 400s first 400 YZ was 76). Depends upon the market but a YZ360B somewhere around $10K (obviously depending upon condition) as there were only around 1100 made worldwide. I think (but coudl be wrong here) the MX400B is actually a 76 model and therefore not eligible for pre 75 but the YZ A and Bs are (with travel restrictors in the Bs).
Hope that helps 
Rossco
The legality of the YZ250/360B in pre-75 is due to some documentation that calls it a 74 model.
Logically, they're not a pre-75 bike - but the argument was presented and accepted, so they are on the list and they they
are pre-75 eligible.
No such docmentation exists for the MX250/400B, so it is has not been added to the pre-75 list.
Anyhow, back to the original topic: Good luck to anyone who has the cash to buy both a 250A and a 250B! Haven't seen any come up lately, but 12 months back, you'd be doing well to get the pair in decent condition for less than $15k.
I've thought of doing the same with YZ125s (I have all of the air-cooleds + an 84 125L), but its hard to justify spending money on bikes that you're not really interested in*, just for the same of 'completeing the collection'.
*J, N, U fail to excite me. Don't ask why, 'cause I won't be able to give a useful answer.
