Our 'cheapest' price for a single is £7.25 including vat, using the 4count kit, cheaper again for large yards and you can claim the vat back if you are a registered business.
Don't equate cheapest with best though. See if the people running the lab are experienced (we have ten years), qualified (we are SQP's). Then there are the extras like can you pay by card (you can with us), are full info and collection kits sent out (yes), are they members of BETA (yes). All these important extras cost a lot to provide but make the service better.