No it doesn't cost that much, there are certain ATOs that were charging that but it doesn't need to cost that much.
To gain the conventional CPL you just need to gap the requirements between the MPL corephase 1 and 2 (SEP training phase) and the requirements for the "conventional" CPL.
So the main shortfall for most MPLs will be the solo time required and the 300 NM cross country qualifier; now MPL courses are different in content so the exact gap will need to be assessed between the MPL course you did and the CPL requirements. Then it is a case of the 15 hour CPL course, so x amount of hours building and the single engine CPL course. Believe MPLs can have the p1 time based on the integrated requirement of 70 hours p1 rather than the 100 hour for modular.
If you wish to obtain a SPA ME IR then the gap in training required does become more marked, but as you have a MPA IR via the 320 type rating I wouldn't see it as necessary if you are only applying for other work on other multi crew types.