It really depends upon the extent of work you need done. You may not known until you meet with them to explain your business and where you are at in development. There will be a standard engagement or retainer agreement with typical contractual language, including specific issues like how they bill, the scope of services provided, any up front retainer, termination of representation, out of pocket costs, and others.
Most lawyers will want the majority of funds to be paid up front and when dealing with a startup, the risks of not getting paid are pretty high. Even if it is hourly billing, they will want at least a few thousand up front, probably closer to $5,000 or as high as $50,000 for extremely complex issues. They may be willing to consider flat fees for specific services or deferring part of their fees or payment until the startup raises money. Plan on at least $2,000 down.