New Crank

I ride a trek 4300 and its not a super nice bike as most of you know. I seem to have messed up the crank pretty bad. I did what I could then took it to the shop. They said that some of the metal might be worn, he tightened it up and said if it comes loose again then it will probably need replacing. [it fell off while i was riding a few weeks after a had it in the shop]

So now it's come loose again and I'm going to replace it. I don't really know how much a new crank is but oh well.. I'm looking to get a new bike pretty soon anyways and was wondering if I should just fix it, or just wait a lil while and get a new bike.