As with all too many online businesses (especially services) there are a lot of rather amatuer 'professionals'. Always ask for references and always
check them.
Look to have telephone contact details for any 'references' that are not well-known companies. There have been
SEO companies who pretended that their own 'pet project' sites and affiliate sites were customers.
An ideal set of references will include at least one client they began working for within the last six months and at least one that they finished working with a year or so ago. That way you get to check that their results are stable, and are capable of repetition.
Price should never be a concern. A good
SEO is like a good accountant - their services may cost a bit, but you make much more back from their services than they cost you.