I have done a good amount of re-grouting and I always charge the customer by the hour on the removal. This only due to the fact that there are tile layers that know how to lay tile and then there are those that lay tile permanently.
In other words the ease with which the grout comes out with varies greatly. Regrouting and sealing can be bid easily. For 800 sq ft I would bid the regrout and seal for one days labor plus material. To make the removal of the grout much easier, albeit not much faster, you can use the attachment sold by dremmel tools. The reason I say not much faster is that you need to take your time with this tool. It will get away from you and score the tiles if you are not careful.
So how much you ask? Mind you this is a sight unseen, off the top of my cap type guesstimate.
Remove old grout - 1.5 days labor plus dremmel bits.
$30.00 per hr. x12= $320.00
2-10 pk dremmel bits @ $18.00ea.= $36.00
Regrout and seal - 2 bags grout@ $20.00= $40.00
2 gallons acrylic mix @ $25.00 ea. = $50.00
1qt. grout sealer @ $15.00
1 day labor to install =$240.00
Misc. expendables $50.00
15% overhead fees $80.00
$831.00 total guesstimate
Bid at $900.00 unseen.
I hope that this has answered your question.
The Home Smithy