RGBHV to YPrPb hookup is not as simple as most people think... just because they are both component video formats doesn't mean you can link one to another directly.
The ATi dongle doesn't just contain electrical routing - inside is an I2C configuration chip that generates DDC (display data channel) feedback. This indicates the present of a component video device to software which in turn configures the R200 to ouput YPrPb. This is only possible because the necessary transcoding and synchronization circuitary is already there on the board (although RGB distribution/difference calculations could be done in the dongle, I'm not sure...). The R300 is slightly different in that it supports YPrPb natively - you can boot with an HDTV connected and it will detect it and switch modes off its own accord (making it pretty sweet for HTPC use etc).
I am not sure about the NV25 - if it could output YPrPb then nVidia would have released a dongle similar to ATi's by now. You can always buy a converter like this...
That's a bit of an expensive example, but even less complex devices that just transcode are still expensive.
Edit - OliverRedfox beat me too it.