Warning. Warning. My con-man alarm is flashing.
First thing to do is get some evidence of actual termites: ask him to show you the live termites. There are a lot of disreputable operators out there and “evidence” is a meaningless term. "Evidence’ could mean that he found a peice of firewood that got chewed by temites befor eit was cut.
Be very careful “evidence of temrites” =/= “termites”. Before you spend big bucks make sure you actually have a termite infestaion, and not just “evidence”. The guy may be gebuine, but many are not. Do not take his word for this. At the very least get another firm to do an inspection and do not tell them in advance what the previous operator claims to have found.
If it’s a wood frame house and the termites have been there for 10 years they could already have done enough damage to require th ehouse be demolished. OTOH they may have done only minor damage.
The point is that you don’t want to let the sit there for another 10 years so they can destroy the house.
Find out what species they are. At the very least you need to know whether you are delaing with drywood or subterranean termites. If they are subterranean termites find out what water source they are using.
Find out what treatment he proposes to use.
Once we have that information we can tell you how effective the treatment is likely to be. Without that information we can tell you absolutely nothing.
Sorry I can’t be more help, but you really haven’t given enough information even for an educated guess. The answers run from “Get your stuff out, burn the house down and rebuild” to “Pay your 800 bucks and be rid of the probelm forever in two days time”.