There are a lot of bike rent outlet in Goa. You may Google for contact numbers and enquiry a few. The rent for a gearless 4 wheeler should be around 400/day.
Check https://www.rentabike.in
They have the best bike rental collection in Goa, You can book online and the service is made available for bike rental in Goa at the airport in Dabolim.
Apart from that the site also shows pickup of bike rental in Goa from Panjim and Mapusa Bus Stand and railway stations like Bike on rent at Thivim Railway Station and Madgaon railway Station. Affordable and cheap is also the right way to suggest rent a bike as the best bike rental in Goa.