Just checked The Guardian's list of gaming charts and they handily publish the breakdown of sales. So, we can see that Football Manager 2012 sold 89% of its copies on the PC, but a decent 11% on the PSP. The portable edition of FIFA 12 gets some 4% of the sales and Pro Evo gets 6%.
FIFA 12 has sold over five million copies, so a guesstimate would suggest that the PSP has sold around 200,000 copies while Pro Evo has probably done around 60,000 and FM12 40,000 with those games still selling strongly. They aren't huge numbers, but considering the job of porting the core code can be done by a small team over a short period of time, the cost must be minimal, offering up a decent level of profit to the publisher, especially with digital sales.
It would be interesting to talk to the guys who get this line of work, and with the PSP still selling 120,000 units a week around the world, there should be lots of decent sales still to come for those who invest and make a little effort. Of course, the big games can still do the job with Final Fantasy having sold over 600,000 copies in two weeks in Japan.