I decided to keep track. To get from 182 to 300, using a no-fail potion, I had to attempt 2231 combines, of which 309 succeeded and 1922 failed.
Success rate of about 13%.
This was using recipes with trivials from 351 to 428, gem combinations using Miragestone, Alaris Gemstone, Harmonagate, Energized Gemstone and Uncut Rubellite. These gems sell for AT LEAST 50pp each on my server (sometimes as much as 200-300pp) so without the potion this process would have consumed something like 110kpp to over half a million pp, depending on acquisition costs for the gems. Minus what could be obtained for the 309 items made successfully, of course.
FYI, 1-182 is relatively easily and cheaply done, following guides here and on EQTraders.