Don't be supprised to find that to re-grout a full bathroom might take just as long to finish as tiling it from scratch, maybe even longer, as to do it propperly you need to remove as much of the old grout as possible.
Then you have the prospect of damaging quite a lot of tiles too, as it only takes a slip and you've just sliced into a tile or 2.
I know of one person who once said to me a re-grouts easy and quick, he just scraped a couple of times into the grout lines making only a narrow channel then re-grouted it up again.
1. It looked bad, and 2. He got endless call backs as the new grout just wasn't holding onto the old grout still left in.
Customers often think it's a quick easy cheapo job, and it isn't, and to charge accordingly can be quite horrifying for them
And think on, if you do damage some tiles and they don't have any replacements left and you can't buy any anymore then you might have a customer demanding you re-tile the whole bathroom out of your own pocket, tiles included, and really you wouldn't have a leg to stand on if that happened, as it was you who damaged the old tiles.
Re-grouting jobs.... I'd have to be desperate to take them on if it was a full bathroom... a kitchen spashback then yeah maybe, but a 20 sqm bathroom... ugh!!!