How much does it cost to file for a legal separation and can you file if you still reside in the same household?

There is no such thing as filing for a legal separation. You can go ahead and separate, even enter into a separation agreement between you and your spouse, but there is no legal separation to file with the Court. May I suggest that, as long as you are going in that direction, you consider Mediation. Mediation is Fast, Effective and Affordable. If you both agree to mediate, it can be done very quickly.

Costs vary depending on state and location. You need to talk with local family law attorneys and find out what they charge.