But as OWG says, it could be a PITA to service - I often find that the PITA customers are the ones paying the least
You'd also have to consider the marketing costs vs the income it would bring - you'd be competing with the likes of City Local, and local type directories, who target the hairdressers, plumbers etc and charge about a tenner a month.
It could be a good 'entry sell' whereby once you have shown the plumber what he/she can achieve, you could upsell to a better site / more services etc.