I wouldn't pay that price. I paid $240 for my Fortress Maximus and it came with:
+ 1 ramp
+ Cerebros
+ Gasket
+ Grommet
It also didn't have the shin guns missing like that one.

I got Spike later on for $80 so in total I've spent $320 on my Fortress Maximus. I have a complete Brave Maximus, so I'm not all that fussed about totally completing Fortress Maximus -- just wanted to have one to cross him off my list.

So considering that that Fort Max is missing both arm ramps, both shin cannons, Cerebros, Spike, Gasket and Grommet - I don't think it's worth $236. $100 is a fair price for that - maybe $150 tops.

Some other things to consider (i.e. things we cannot see from the photo):
+ Are the two small ramp-doors (leading to the lift) both present?
+ Does the lift & launcher gimmick still work?
+ Are the two dual-arm cannons present?
+ Is the prison door present?
+ Is the cockpit window present? (where Spike sits in vehicle mode)
+ Are the two large body cannons still working (i.e. able to swing in and out okay - no damage)?

I think these are all fair questions before deciding how much one should pay for this toy - because as you've said, hunting down individual parts is going to be time-consuming and costly, so if the base body is missing any of these things then I think it's reasonable to ask for further discounts. If the seller won't, then just move on IMHO.