regrouting

I have a shower floor with 20 sq ft. - 2" square porcelain tile. The entire floor needs to be regrouted. We just bought the Fein last night so we don't have any idea how long it will take to cut out the old grout because we don't know of it's efficiency. Even at $10.00 sq ft $200.00 just doesn't seem enough. We want to go hrly. Any ideas how long it would take?

Thanks, Josi