I haven't checked the 2019 price changes yet, but US retail for the 1st stage kit is $40 and the 2nd stage kits are $23.60. I assume you will need two. Then there is the labor cost, which will vary widely. Just for comparison sake, the actual Aqualung retail price for servicing a 1st stage + primary 2nd + octo is $115, but that is very much the high end of the spectrum on service pricing.
One other note, in case it matters to you. Below are photos of the current XTX200 first stage, and the current XTX100 first stage which at some time in the past used to be the XTX200 first stage, I think the transition was in 2006, but I can't be sure.
The major difference between the two is that the crown orifice in the new version is removable/replaceable, whereas in the old version it is machined as part of the body. It is exceptionally rare that one gets damages and needs replacement, but since you are buying used you should be aware. Especially if it looks like someone in the past might have managed to contaminate the interior of the 1st stage with any sand, grit, etc.
Also, XTXT200 prior to 2013 will not accept a 5th port upgrade kit. If you don't know what that is, then you very likely don't need it.