Paying any more than around $20 for a 30ml bottle isn't going to happen in my case. I think around $15 for 30ml is where the cutoff should be, but I'll go up to about $20 if need be. The shipping also factors into the $20 per bottle don't forget.
$15 bottles of 3 different flavors plus shipping is actually $18.34 per bottle without taxes. Of course the more you get the better the price, but there has to be a limit.
I find DIY a pita to be honest, but keep a few bottles around to mix into the premades I'm not happy with or find too strong\weak.