As far as DJ compilations go, there is no such thing as a Cheap way of building up your collection really.
IMHO, Mastermix are the best collection(s) to start with, what sort of music are you looking for?, is it latest stuff or stocking up on the oldies for those Private functions?.
If you are looking for the classic stuff, then the Mastermix Box sets are a good place to start. I would start with buying the 60's and 70's Box sets, which both consist of a 4 cd compilation set. you won't get every track from these era's but you will get some of the more D.J based popular ones

and it will at least give you a running start.
After the box sets then look at the "Classic Cuts" compilations - especially the "Party", "Soul" and perhaps some of the "Disco" ones. You are not going to get an entire collection overnight, but we will be happy to point in in the right direction, with which cd's of the series to buy first. Also have a look at the other "Playlist" Threads to get a good idea.
The Mastermix website is
HEREFrom reading your posts if i'm really honest with you 3G, then I think you do need to spend some money on building up your cd collection first, before buying more equipment

. (This is why I'm a D.J first and salesman second

- LOL)
If you need some idea on what to buy from MM first with regard to the compilations then let us know!.