I'm quite surprised you were able to get a decent dual stage setup for $200 all in when the burkert valve alone is $100 shipped (with the power cord, and new), and a worthwhile needle valve is anywhere from $75 to $200 used. That is nearly your total, and thats without the rest of the apparatus.
The only thing I wouldnt buy used in these setups is the burkert valve. That's $100 right there, maybe $80 if you're lucky. Getting the rest together with decent used equipment that will last longer than the carbondoser, for less than the remaining $100, is very rare indeed.
This is my breakdown.
Victor VTS-253B regulator - $90
Fabco NV55-18 needle valve - $35 brand new
Burkert solenoid - $50 brand new aquarium plants
Misc fittings - $25 mcmaster carr
I did luck out on the regulator but deals like that can be found. I know of people who have scored on pretty much brand new Matheson SS ones on ebay for not much more. You do not need a $100 needle valve. The Fabco is more than good enough for our needs.
If I wanted to save a little more I could have gone with Clippard mouse solenoid but those are harder to find.