It will primarily “cost” them however much medierra pays his employees - which we don’t know. We can make some guesses…
…but this amount varies widely depending on the role, or roles, each individual serves and their experience. You must also factor in how many employees he would assign to work on it “full-time”, but I imagine there would be some overlap - the artists for instance would likely be working on multiple projects simultaneously.
The amount of time it will take them to complete an expansion, based upon how much time it took them to complete AoM and FG, respectively, is roughly about 1 and a half years for a “full” expansion that features content, gear, masteries and new features.
Anyway, all indications are that Crate are not currently interested in making a 3rd expansion and they have made this clear for quite some time now and I highly doubt there is little you could ever do to affect that. They certainly do not need any money you might attempt to “raise” to persuade them with - as a company, Crate is rich off the success of GD and can easily fund any further projects they wish without needing to Kickstart or take any community-driven efforts to fund them.
As for inspiration - here again I very much doubt you are going to sway them as they made their minds up some time ago that it was time to put effort into other endeavors and all developers are currently invested in working on new games - there appears to be several projects they are working on currently and you need to keep in mind that this is a small dev studio with around 13 or so devs total (tho they are hiring for a few positions). What this means is that they must manage what they are working on tightly and as noted above they are very much busy with other things now.