Sadly the only two options I know of that work with the FLIR Vue are Pixhawk and the system you referenced.
The creation of a specialist GPS adapter for the FLIR VUE took someone some time in development, so sadly the price will be high. It may well be based upon a ATMEGA AVR microcontroller but that is a moot point as creation of the software loaded on it will require in depth knowledge of the FLIR VUE communications port and functionality. Sadly I cannot help with those as I have not worked with Mavlink or the FLIR VUE.
300+ Euros may seem like a lot if money, but how long will it take you to develop such an interface ? For 300 Euro's you get a proven system that may be used immediately.
The world of Drones is expanding fast. They are still relatively expensive for survey capable models and the FLIR VUE PRO is also an expensive product. To a surveyor, 300 Euro for the GPS interface would not be that much of a surprise. Hence that is the sort of money being asked for the interface. A good quality, light weight, self contained Serial GPS can cost 100 Euro alone.
None of this solves your problem though, and for that I am sorry. You may get more help with this challenge on the better Drone forums that deal with creating and modifying Drone electronics packages. There are already Drones with built in support for the FLIR VUE so maybe DJI will provide support in the future. Sadly that could be a long wait however.
In your position, if the need is justifiable, I would buy the module you referenced and and consider it all part of the thermal surveying costs overhead.
Fraser