i was just wondering what is a good amount of bonus healing for a 21/40/00 priest?

FOA, 21/40 is not an optimal build.  I assume you got Divine Spirit but not improved?  I would highly recommend considering either a 23/38 IDS build or a 20/41 CoH build.  I have posted builds for both in in various threads in the talent forum.

As for Bonus Healing, it's not a simple question.  It entirely depends upon what content you're working on.  If you're doing Karazhan, 1200-1400 is a decent place.  If you're doing Gruul/Mags, you want more like 1600+.  Early SSC and TK need more like 1800.  Hyjal and BT easily require at least 2000, probably more like 2100.

The thing to keep in mind is that Priests are the least gear dependant of all of the healers.  Pallies pretty much spam FoL until they're OOM.  Shamans spam CH.  Priests depend on playing with the FSR, so a highly skilled Priest with poor gear can keep up with a well geared healer of another class.  Similarly, a well-geared Priest lacking skill can lose miserably to a somewhat lesser geared healer.

The other thing to keep in mind is that as long as your targets aren't dying, you're fine.  If the tank has enough gear to let you be a little lacking in healing, then that's okay.  If the DPS hits hard enough that you can lack in longevity, that's okay.  Big healing numbers don't mean anything if the heals aren't on the right targets at the right times.  Beyond that, the extra stats just give you some breathing room for mistakes, or unexpected things (like a chain of crits, or someone pulls aggro).

My goal, as a healer, is to be running out of mana when the fight ends.  Obviously, for fights for which I'm overgeared, I'm not going to gimp myself to do that.  If we're on a progression boss, and we're dying at a certain point, and I have tons of mana, I consider trading some regen for +Healing for that boss or possibly some rearrangement of the groups.  If I'm running out of mana early, I have to pay more attention to my cooldowns (HC, IF, ESM, SF, Pots) and consider using particular consumables (as I almost never bother on non-progression bosses, and even many progression bosses, the issue isn't my healing output or longevity).  Bottom line, if you're killing progression bosses and have to chain chug mana pots and beg for Innervates just to make it through, reconsider the heals and gear you're using.  If you end up with 40% mana, adjustments can probably be made to either up raid DPS (by rearranging groups) or increase your healing output (if a lot of people are dying or getting close to dying).