The cheapest I can do it for is as follows (inc. shipping) 
 
$98 regulator with solenoid 
$45 adaptor 
$40 per sodastream bottle new (around $20 second hand on TM) 
Diffuser, bubble counter, check valve etc anywhere from $20 up to $50 usually 
 
Refills cost around $12 and last a 60x30x30 high tech tank around a month. 
 
How big is the tank you want it for?