I guess it's value is relative to what you value your time as, even if it was $1000 it would pay for itself relatively quickly in my case. not to say achieving the same result for less money is anything other than a great idea.
150 usd for controller and pump is a bit too much. It can be done with about 30 usd if you use arduino board as controller and a cheap perisaltic pump off ebay. The PH probe is probably the most expensive bit of the system.
I bought a p63 and spoke with Hach tech support and the Model # p63a1n1a1a1nnn which he stated the fourth place (a) is a denotation for the Differential Probes. The manual shows the connections for a Combination probe BNC type. I have not powered this unit up yet but the manual also shows that you can switch which probe you are using either a Differential or Combination. As of right now I only have a access to a Combination probe with a BNC connector. I was going to cut the connector off and wire it in but I want to know if this will work first. I do have a Differential probe on the way but did not want to spend $$ as I have access to a lot of BNC combination probes.
Also what do you use for your reference solution in the differential probe? Standard 7.0 reference solution like GH 7.01 or their $$ reference buffer