XBT is going through some pretty crazy swings right now, as is the BTC/HVC exchange, so take this as indicative rather than authoritative..
Current BTC/HVC: 0.00004570 (mintpal)
Last 24 hours average hash rate on pools: 67,470Mhash/s
Last 24 hours blocks found on pools: 675
Block reward: HVC 550
24 hour reward = 675 * HVC 550 = HVC 371,250
24 hour reward/Mhash = 371,250 / 67,470 = HVC 5.50244553
24 hour reward in BTC/Mhash = 5.50244553 * 0.00004570 = BTC 0.0002514618

So if you have a card doing 15MH/s, the past 24 hours would have gained you (very roughly) BTC0.0037719264.
( Note that it'll depend on pool fees, your pool's luck, the aforementioned fluctuating variables, and possibly the alignment of planets. )