I looked at the Avis
site, maybe a good starting comparison with other quotes, for eg a seven-day hire Hua Hin - Phuket for a small car Toyota Vios 1.5 Manual includes: Local Tax, Collision Damage Waiver, Vehicle Registration Fee, Unlimited Mileage, and No one way fee is charged for this rental, came in at 8225 baht. Approx 170 GBP
GPS is an extra 200b per day.
Not recommending that company, but personally I'd stick to one of the international companies even if they may cost a little more, I'd tend to trust their insurance/accident cover, and vehicle condition, more than a local business.
There are no 'internationals' in our area, so if necessary I have rented from a local; had one almost
horror-story when I went to collect a vehicle and started it up and the clutch 'blew' as I went to reverse out of the car park, right in front of the owner. Declined offer of another vehicle and made other arrangements, at a guess I would have been held liable/blamed for the clutch repairs if it had died when I'd been 'on tour', a lucky escape as had provided credit card as security. Never again.