50k for a job well done, I would think. I suppose you want to rent it out, so make sure the bathroom is full, with plumbing done right, there is a back door so you don't burn your illegal tenant to death, good lighting, and at least 6 foot ceilings, although 8 ft is really legal. Put in a fridge that is very easy to hide should inspectors come, even better, don't let them in in the first place. Don't put in a gas stove, only electric as this is quickly concealable. Of course, it will never be legal, but this is New York, for God's sake at least make him safe while he lives in a cellar, which is why you NEED carbon detectors/smoke detectors while his head is near the furnance than prisoners are. And lastly, install fire estinguishers.
For all your trouble, your 50k investment will cost you 350/month, and you can rent it out for about 700. That is, unless you rent it to your sister in law, who left owning us $3150 when her rent was only $350/mo and she had use of the washer/dryer. On that note, make sure that the washer/dryer is accessible by you but not by them, and don't sign a lease, if you do mention that they are only roomates not lessees. Find a reliable tenant, because someone worse than my sister would take you to court, get 6 months living free, and the court would require you to convert it back to an unlivable basement, with penalties.
OR better yet, do the conversion, and live down there yourself, renting out the top, and hopefully they cover the mortgage while not trashing your house. (Hopefully your wife and kids won't mind too much). So if you are going to take this route and live downunder, it is not the way to surefire wealth, but if you are single w/o kids, it is a way to greatly reduce expenses, build wealth, and watch other people enjoy the house you saved up so much to buy.