I don't think there is a consistent pricing formula for metamagic rods. The closest simple formula that I can get for the lesser metamagic rods is 2,000 gp x level adjustment^2 using only the SRD lesser metamagic rods as data points.
The formula gives 2,000 gp, 8,000 gp, 18,000 gp and 32,000 gp for Extend, Empower, Maximize, and Quicken respectively, which is fairly close to the listed values of 3,000 gp, 9,000 gp, 15,000 gp and 35,000 gp. So, for a lesser Persistent rod, I'd start at 72,000 gp, and probably go a bit higher, to 75,000 gp or maybe even 80,000 gp.
Just a word of warning: metamagic rods, Persistent Spell, and custom-made magic items can sometimes provoke extreme reactions from some of the posters here, so don't get too upset if people come into this thread just to say that they would never allow such an item, and that only a munchkin would want one.
