£99 minimum charge (and he's sticking to it) sounds like your not wanting working..
You do a full house and charge say £250. Your happy and customer thinks thats good value. A few months down the line the customer calls again and says they only need the lounge doing. Do you say "sure, that'll be £100"

The first thing I'd say if I was that customer would be oh right, i'll discuss it with my partner and get back to you

I'd then ring around and get someone that charges a "fair" price for the work that i'll start using them in the future and you've just made your business that little bit smaller by sticking to your price.
But hey ho, atleast you can puff out your chest and feel good about sticking to your principles!