I'm developing a Mobile App with the help of Developer.. at the end of the project I'm asking them to submit the source code.. they're charging again Extra for it.. is it correct way ?
As with many design projects, this should have been negotiated up front. The typical cost for source code for software or raw design files for graphics work is about 3x the cost to simply deliver a finished product. When you aren't guaranteeing them more business by letting them retain the intellectual property so you need to go back to them for updates/changes, then you could take their work anywhere. This is why source files for designs are always more costly than simply the finished product. Showing you how it's made is valuable.
This issue is as old as sin, can you profit from gaining access to the source code? If that don't answer your question, then determine whether to wait out a resolution and the P/L from that advantage, and the issues of resolution to your ego, that will cost you, no matter the consideration! Now if you can provide the equation, will give you an unequivocal answer readily!
Well, this isn't usually the case. If you did not have an agreement in place, it's difficult to fight your case.
Usually, if you paid him on an hourly rate, he should handover everything he worked on for the hours he charged you.
In your case, what I recommend is...
1. Get on a call with him and tell him that you are talking to your lawyer and will make payment for the source code only if your lawyer advises so. (this will create a little bit of fear in his mind if he is trying to rip you off)
2. After 2-3 days, tell him that your lawyer has decided to file a case against him.
3. Hire a local lawyer (if it's any developing country, it wouldn't cost you much. Hardly $100 if it's India), explain your situation and ask him to send a legal notice to your developer.