buckworks - 2:32 pm on Dec 3, 2012 (gmt 0)
It really depends upon how well they're built.
This is VITAL. If converting to WP is done by someone whose SEO knowledge is weak, rankings can suffer a lot of damage.
I'm presently involved in helping to rescue a site that lost a big percentage of its organic search traffic after a very bad WP conversion a few months ago. The site looked nice, but on the technical side many things were poorly done.
Pay what it takes to get someone truly competent.