It doesn't matter whether they are using a new engine or not. Since we don't specifically know either way what their tool sets are, we only need to look at the more important claim that it is "cheaper" to develop on PSP because they have an engine. How do we determine cost? Development time multiplied by manpower, correct? Birth by Sleep has a sizable staff comparable to most PS2/Wii productions. Development started in early 2007, and it was announced at TGS 2007. Development completed at the end of 2009, and the game was released in Jan 2010. That's almost 3 years of development for a PSP game.
Agito XIII switched from a mobile phone platform to PSP in 2008, it's still not out, with no signs of coming out anytime before the end of this year. That's another 3 year cycle. I do not doubt that Agito XIII has even more staff working on it compared to BbS, because of the importance of the brand to S-E, and also because it will need to support network functions which go beyond what S-E has done on any of their PSP games so far.
Also, Dissidia and MPO didn't really sell "well" in the US. When the Japanese sales eclipses the US sales significantly for a game in a major worldwide franchise, it is not good. We'll really have to see how BbS and PW does in the US before speculating any further though. I don't see a point in suggesting that games should be ported to other platforms for financial reasons before the games are even out.