Lots of money !
Most GA maintenance companys have not the first idea of customer service and communication and so when problems arise the customer is usualy the last to know, turning a job out on time is likely to never happen as there is no incentive of the maintenance company to do so.
I run a number of aircraft on lease and largly cover my companys maintenance so aircraft get fixed ASAP ( a week or so back a vac pump failed on a weekend and it was replaced within 2 hours of it being reported) but I have no end of trouble with contractors not meeting the time goals that they have set dispite the fact that invoices get paid almost by return, following a few years in the business the companys that have performed constantly well for my business and I am not conected with in any way except as a customer are Lees avionics, Phoenix avionics(for radio box repair) but so far the best service award has to go to Airpower who have overhauled three engines for my business in the last year or so two of these have been unschedualed and the turnround time has been seven days in all cases.
For parts supply Airparts at Booker are very good but for the items such as lubricants and chemicals that they don't stock I have had good service from LAS in Devon.
Most maintenance companys will give you a fixed price for a LAMS maintenance check added to this has to be AD's, defect rectification and parts, of late I have been shocked to find how one owner I know was taken to the cleaners by a maintenance company who told him that "one spark plug was faulty but they could find out which one so they changed all eight".
I have been in the past contracted to a flying group to "keep an eye" on there maintenance contractors, this helped both sides because my just being in the loop kept the maintenance company on there toes but I also recomended some preventative maintenance to the owners that the company felt would be seen as work generation, they were of course happy to carry out the extra work.
If I was a new owner I would get a licenced engineer on my side on to look over the work being done, not choose the cheapest quote and only use companys that other owners had recomended, I wish I could publish a hall of shame but I don't want to see PPrune in the courts