Dr Foster has them on sale for $34...
http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+3579+3608+12320&pcatid=12320
Big Al's has them for $37...
http://www.bigalsonline.com/BigAlsUS/ctl3684/cp19169/si1379661/cl0/marinelandpenguin350bpowerfilter
I would show those two links to your neighbor and then offer him $30... unless you want to do him a favor and give him the full $40...
I've had several Pen 330/350 filters, they aren't bad. I definitely suggest you learn to make or rebuild your own cartridges as buying replacements gets expensive fast.
Personally, I would buy an Aqua Clear off of Ebay if I were looking for a good HOB for cheap, but buying it from the neighbor helps him out and gets it in your hand immediately.