I don't know the answer, but I'm guessing the cost is $0. I'm not using the term ROM in the purest sense. I'm using it in the context of how the image of a ROM.
Over at the XDA forums there have links to custom roms as well as Factory ROM. You have to download the ROM and then do whatever install process is necessary to update the ROM.
Usually the work required to ROM is not that much or difficult. It's really understanding the instructions that is hard. I've ROMed my phone and Nook and they all took about 20min. However the research took hours.
Here is a link to the XDA site where you can find better andwers to your questions.
http://forum.xda-developers.com/forumdisplay.php?f=844
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