I find the difference in price usually relates to how they actually prepare and carry out the flush
A proper flush should be preceded with the system being drained and flushed with mains pressure and then a sludge/scale remover added to the freshly filled system, this should be allowed circulate (preferably with hot water) for at least a week, longer even better
Then on return, the system should be flushed with a flushing agent, again preferably with hot water, all circuits, rads and boiler, should receive an individual flush (all other items isolated) for a give period in both directions.
Once finished and checked the system is refilled (possibly neutralized if something like ds40 chemical is used) the a good dose of a reliable inhibitor should be added, and my preference is to also add a boiler silencer.
The system should then be tested for sufficient dosage of inhibitor, and labeled so this can be checked yearly on an ongoing basis.
But that is all just MHO, and you wont get it for 300euro!