Fair price is contingent upon many things: age, condition, trust, budget, current market value, etc. Per Atlas Pro Audio (the first link across which I stumbled on Google), a new pair of 603As + KA200 cardioid caps runs around ~$2,100. I assume that's MSRP. Actual street value is probably ~10% less, as a guess. After that, it's up to the other factors I mentioned. I usually figure a 10-30% discount off street prices for used gear, depending on age, condition, trust, current market value, etc. FWIW.