The thing about exchanges are that they are not easy to do right. You can make a decent one, but without the proper network security it will get hacked very easily. If you have a small budget, you should find another market to get into.
If you still do not want to heed that warning, the only open source script i know of that is worth much is an opensource ROR script. some chinese people made it. it seems legit, but i haven't poored through enough of the code to see if they have hidden a vulnerability in there in order to have potential victims who use their script. the name of it escapes me but you should be able to find it by googling open source bitcon exchange
If you want something easy to manage, you can try
http://www.bex.io/ they manage all the security and coins while you manage the site and the legal requirements. don't know how legitimate they are.
I myself have written a fairly advanced trade manager (trade engine), but i am not interested in sharing the code. I have built two separate exchanges already, and i am not happy with either one of them. I'm currently building a 3rd version but its nowhere near being done, and as long as i am detained with my real job, i likely won't make any progress on it without monetary incentitive, ie a potential buyer.
