I don't know what an MRO is, but I am an owner and, for a given type of plane, you should be able to get agreed prices for the labour, plus any parts.
On a new plane, the resulting cost is only slightly over the quoted price (due to consumables etc) but on something much older the maintenance cost can end up in a different universe to what you might expect. Someone next door has just spent 10k on an annual, 25% of the value of the whole plane which was 15yrs old!
I pay GBP 300.00 for a 50 hr check, 1200 for a 150hr check, 1500 for an annual and a bit more for the star annual (every 3yrs). This is for a Public Transport CofA plane; on a Private CofA you have just the annual, and same if the plane is on the N register (the intervening servicing still needs to be done as required by the mfg etc but you can do it yourself). These prices are for a complex retractable; on a Cessna 152 it would be about 2/3 of the above figures perhaps.
The cheapest way to run a plane is to become a licensed engineer (or an A&P if N-reg) and do your own maintenance. The GBP 300 cost of a 50hr check is ludicrous for what they actually do...