We average out at £35 per room if on site and use chemtreat plus and/or diatomaceous earth. Bedbugs £75 per room including steam treatment and vacuuming (generally we do the whole house for bedbugs and quote accordingly).
What insects are you trying to kill?
The law requires that those born after 31 December 1964 who apply professional (agricultural, horticultural, amenity, industrial and forestry) pesticides commercially must hold a certificate of competence.
Not sure how it applies to indoor treatments, guess it won't matter until client (family or their pet) has an asthma attack or allergic reaction. I would advise you to check with your insurer first to to be safe.