I just had a great run ruined by saying to myself "the blood bag HAS to spawn THIS time!" and spending about 10 hearts + soul hearts worth of donations and getting no blood bag. 20-ish donations and no bag. I usually see it spawn in less than 10 tries, but apparently there is no set amount you can spend and be sure to get one, so be warned!
Anybody want to guess on the approximate odds on a bag per donation based on their experiences so far?