Is $500 to paint door frames and stairs a fair price?

My attempt to paint over door frames didn’t go well. I applied Zinsser Bulls Eye primer over wooden door frames that were painted with gloss, planning to use eggshell and satin paint. The primer didn’t stick well because I did not sand the gloss first. Now I need to redo the door frames and also our stairs, which have a similar issue. I need step-by-step instructions to avoid rookie mistakes. I've received professional quotes but prefer not to spend around £500 at the moment, as we still have a lot to fix in our first house.