what is broken on your derailier?
If the spring is busted - you can not use it to tension the chain. If it just does not shift correctly - you have something to work with. It is possible to set up the der. to act as a tensioner for the chain.
First thing you need to do is determine which chainring/ cog combination you want. This will depend on the terrrain you are going to ride. Once this selection is made - disconnect the shock from your rear suspension and, probably with a friends help -more hands make it easier, move the rear triangle of the bike through its complete range. While doing this, determine the longest length of chain you will need. Shorten the chain, run it through the wheels of the broken derailier, around the chainring and cog you selected. the top wheel of the derailier needs to be directly below, aligned with, the cog the chain is on. Pull the derailier cable off the bike and run it through the hole in the derailier that the cable went into and secure it with the clamp. The barrel at the end of the cable should be down in the hole and you are only using about 3 " of cable - cut off the excess. Adjust the cable length such that the upper wheel on the derailier is aligned with the chosen wheel cog. retighten the cable clamp - you can fine tune the position of the derailier wheel with the barrel adjuster.
I have used this setup sucessfully - it may take a little adjustment, but it will allow you to ride your bike.
It sounds like $$ is tight. If the derailier spring is broken - most any bike shop will have a box of old derailiers - they would probable give you one for nothing. If you do not want to cut your derailier cable short - to use it when you get the $$ to replace you derailier - the same shop will have a piece of discarded cable they will give you,also. Just ask.