Once upgraded to "+CARGO 1" fulton, you'll probably notice that now you're able to recover weapon emplacements, as far as I know only the MG turrets and mortars. There is more to fulton, as you know, but isn't available until "+CARGO 2" is researched.
Not to worry! Cargo 1 should be fine, although when you obtain cargo 2, this process will increase you GMP depending on what you extract. Once cargo 1 is ready to go, simply use the fulton to extract every MG and mortar you can (some are sheltered, and cannot be extracted by normal fulton [I probably shouldn't say in case of potential spoilers]). However if nothing is obstructing from above you should be fine. I recommend only using if the chance is 100%. Sometimes you can get 80-90% or lower but it is quite risky with emplacements. When you extract them, you'll notice that the finances add to 5000 GMP. That may seem big, for a gun that sits on the ground, I know.
But once it reaches mother base (you'll be prompted at the iDroid notifications) that's when the GMP will be made. Open the iDroid by pressing start, or tab, if you're a PC gamer, and navigate to the left side tab, where you go to manage mother base's staff, facilities, etc. and select "Resources".
When you see this page, find the tab with weapon emplacements, and take a look for the GMP you can sell them for. Remembering that the mortars and MG's were 5000 to take, you're able to resell the mortars for 9000 GMP and the MG's sell for 12000. That indeed has profit, and to me is worth extracting all the equipment I see!
Now to expand on this, you probably remember when I mentioned that cargo 2 would help the profit. This is indeed the case. When you research cargo 2, you will now be able to extract containers, vehicles, walker gears, and AA emplacements (cargo 1 cannot, I believe). Using cargo 2 has finances of 10000 GMP, but if you extract mortars and MG's it will remain 5000 GMP, so no worries there. Now when selling vehicles, try to keep in mind that you may need some of those for mission purposes. Personally, I only sell the cargo trucks off the bat. They don't prove useful in deployment for Snake's advantage to well and also give a nice profit. To extract a vehicle, remember it costs 10000 GMP.
If I am not mistaken, the cargo trucks sell for 22000. If not, it's close to that sum. Around 20000. Big profit, eh? Let's try to put this in perspective: Say you only have exactly 10000 GMP. Extract a truck, now you have 0. Once arrived at base and sold, you now have 20000, which is basically double of what it costed to extract.
A huge tip: When spotting a cargo truck, my mentality of it's non-useful addition to Snake, I naturally just avoid it. But the better thing to do. is to clear the area, so no guards see it being stolen, and always make sure you're clear weather wise. They may be useless in combat or sneaking ops. However they make good money.
But personally, if I start to have a lot of off road jeeps, I do sell some of those, too. Never all of them. They're key for a lot of scenarios. I hope I helped those struggling in MGS V financially! Cheers to you all, happy farming!