15 mg to me sounds pretty high. I've been taking 7 mg daily for years. Another on this forum, five weeks post-op, is taking only 2.5 mg./day.
It seems to be a matter of what dosage actually brings you within range. I'm not aware of any studies that say a high dose of coumadin can cause other problems in addition to just the anticoagulant effects. (In other words, even if 15 mg is the correct dosage to bring your INR within range, is the coumadin hurting you in other ways?)
As others have pointed out, aspirin has a different effect than warfarin, so one can't really replace the other in its effects. In fact, I'm not sure that the effect of aspirin will even show up in an INR test, since it effects the platelets and not the actual clotting time.
FWIW - although aspirin is actually pretty inexpensive, if you can take uncoated aspirin, you'll find that baby aspirin is also 81 mg and may be a bit less expensive than a coated 81mg pill. Or, if you want to get really cheap - you can break a 325 mg aspirin into quarters and take 1/4 a day (for what should cost you less than a penny a day).