It is. I paid $400 for my primary and applied to 9 schools. So the calculation above is off. For 15 schools, I'd go for more like...
Primary:
$160 + $30(14) = $580
Secondaries:
$100 * 15 schools = $1500
Interviews (2 driving... I can only drive to one of mine, personally, and 4 flying):
Driving = $60 for gas (roughly 2 tanks in my car)
Flying = $300 round trip (most of mine are more) * 4
Hotels (you can sometimes get out of this) = $50/night * 4
Total = $1460
And, of course, you can add your MCAT ($210), maybe some prep materials (not a course, I spent ~$180), perhaps a suit (we'll say $150).
Total cost: a little over $4000, which I think is about average.
Depends on where you live, where you applied, how strong your stats are (how many interviews you get), etc.