Cheap and lockers don't go in the same sentence.  However, they actually are some pretty reasonable lockers out there.  If price is a concern then go with a lunchbox style locker.  THe best on the market right now would probably be the aussie locker and check out the other thread.  The other option would be a powertrax.  The aussie is about $350 but check with HBZ for exact pricing.
As for a full carrier replacement locker there are lots of choices.  Depending on where it is a daily driver or not and how you drive, you have the choice of either selectable (ie ARB) or automatic (detroit).  An ARB will run you about $1100 for the locker plus about $350 for the compressor.  A detroit I believe is about $750-800, then install is on top of that at about 7-10 hours.  Other selectable include the Detroit Electrac, which is a limited-slip when off and a locker when engaged, Teraflex has one out and the OX locker.  The Electrac is probably the most expensive with the OX a little cheaper.  Detroit are super strong and maintanence free.
Another choice would be a spool or mini-spool.  I spool replaces your carrier and a mini-spool is just like a lunchbox locker, it replaces your spider gears.  Spools are cheap $150-200, but your axles are always locked together and this setup is not very road friendly.  Also it can put a lot of stress on your axles.
Finally, you last choice would be what is sometimes called a Lincoln locker.  Basically all it is, is you weld your spider gears together and presto, you have a spool.  Super cheap especially if you have a welder or know someone that does.  The downfall is it preforms like a spool on the street and it is on as strong as your welds
Now if you have a YJ with stock axles, especially the rear D35, 33s, and a locker there is a very good chance of breaking axles, especially if you have a heavy foot.  Just something else to keep in mind.
Hope this helps