The new Who seasons were actually all on sale in last years Amazon's daily black friday deals. All the seasons were around $20-$25, which is a really good price for them.
As for the older "seasons", unfortunately you're going to be fairly out of luck on most of them.
The thing is with the older "seasons" is that there isn't really a full set for each of the older series mainly because a lot of the episodes from the older series were lost (funny huh), and some of these lost episodes have been remade as the flash webisodes that some of you Who fans might be familiar with, but it is because of this, many of the older seasons are actually incomplete. In general you would have to buy each of the who series by a single or two DVD pack at a time. There is no super pack for any of the older Who series. I've searched far and wide everywhere, but there just isn't one unfortunately. And I had to find the majority of all of them off BBC, and many of them you couldn't even track since they so old and not on BBC anymore, so I had to look elsewhere.
If you were planning to collect for the sake of completeness, it would probably be easier to just start with the new Who series anyways.
Anyways, best of luck on your search for Who.
*Doctor Who theme music*