This is a minefield. Its also in relation to a LOT of money, more money that even a medium sized business can put aside in an
SEO budget. They rarely realise the importance, shame.
Still, one couldnt expect the average Joe to afford the services from a reputable company and every other company is almost certainly a risk.. a risk that would certainly make me feel uncomfortable though i have seen an
SEO company or two offer a performance based quotation.
In the short-term, I'd suggest to small to medium sized concern that they spend their own time on acquiring good quality links themselves. There is no substitution for hard work and a lot of money can be saved. Find those related sites and beg for a link from it.