The only place I've ever refilled was at Liquor Locker on Morena Blvd. Its about $15 for a 5lb tank. I typically just swap at west air though, just use the fill if I'm in a pinch or need off hour fills. If you got o home brew mart, they have a list of places that fill.
goto airgas on clairemont mesa boulevard....it's on the same side next to the county court house. Can't miss it. I think there's a coco's across the street from it.
They've got a good selection of craft brews. 5lb refills are $16 I think. They also deliver kegs, CO2 and have a $10 or $15 line-cleaning service. Never used their delivery or cleaning services though.
I use to go to airgas, but stopped when i realized they were making up the prices as they went along. At first, it was always around $18 for a 5lb swap. Then it was $24, then $21, then $26! I asked the guy why it was so much more and he said "retail price is $30, I'm giving you a good discount." So I switched and haven't looked back.