Assuming I got it to the lowest cost possible (180k per 2500 shots) is it more cost effective than training range with a r c bow with braod bolts? Does it give faster xp?

I think it's more cost effictive to use broad bolts/rune crossbow because while the crystal bow is 72 gp per shot, you can pick up most of your broad bolts, making it cheaper.

Not to mention you hit harder with the cbow and its accuracy remains a consistent +90 (98 if you're using zammy book, which you should).

 

However, you can get around the accuracy loss with the crystal bow with the double bow trick, but that requires a fair amount of attention.

Crossbow is better.

Sadly, the crystal bow has been outdated by the crossbow =\