I don't know if the older ones are cheaper but I wouldn't start with EDF6, though. Maybe 4 or 5. Or even 3. While it's normally the doesn't-fucking-matter-est series ever, 6 has a particularly batshit time travel story/structure that I think also calls back to earlier ones a bit?
Plus since they're all almost the same game with minor mechanical improvements if you start with an older one you'll have a bunch more to look forward to if you want more.
You're getting a crash course now.
The six mainline EDF games can be broken into 3 continuities. Outside of each pair of games they have no relevance to each other outside of easter eggs:
Monster Attack and Global Defence Force (PS2/PSP exclusive, fully emulatable)
The original two games released to PAL regions as budget titles. Featuring angular blue and yellow wannabe-mecha EDF vehicles and children's toy purple and green UFOs. Monster Attack stories the first invasion and GDF stories a second return invasion. Monster Attack is extremely barebones but a novelty nonetheless and GDF is a lot of fun and marks the debut of classes and the Wing Diver way early to the game. But it runs like shit and was so low budget they didn't translate the game to English at all. And without any AI allies it's an oddly isolating game.
Earth Defense Force 2017 (360 exclusive, PS Vita rerelease with more features, emulates perfectly via Xenia) and Earth Defense Force 4.1 (
Steam)
Reboots of Monster Attack and GDF. 2017 is the first game to have released in the US and is basically the pinnacle of the barebones most basic EDF gameplay there is. Starring bubbly chrome bizarre-o alien enemies. An excellent time waster but players used to the classes in the newer games will probably find only being a Ranger boring. The PSP version added Wing Divers. 2017 ran like dogshit on actual 360 hardware but emulated on Xenia it runs at a constant 60 fps. 4.1 continues 2017 with the second invasion again and is my personal favorite of the whole series, with all of the classes from the later games present but without a the newer mechanics and annoyances I have complaints over in 5. Has the coolest final boss ever that literally blots out the entire sky.
and finally EDF 5 and 6 (Modern consoles and PC) are yet again full reboots once again retelling the first invasion again and then the re-invasion again, again. I'm not a big fan of these newer games because I feel like they lean way too far into hamming things up to the degree where it starts to feel forced, though it is still funny. They also make a lot of unnecessary mechanical changes that sound sensible on paper (namely damage falloff for bullets and explosions) but don't translate well to gameplay (Most assault rifles and shotguns deal little damage at 100m in a game where you are fighting enemies taller than that, explosions were good in the old games because they evenly dealt their middling damage to all enemies in a radius, now with falloff their up-front damage isn't enough to compensate and the result is they're actually very weak in general.). Wing Diver has an absurd damage output but basically every weapon of hers has some kind of annoying fucking gimmick attached to it. 5 also has the retarded weapon upgrade system that makes new guns worse than guns you already had and completely rapes the whole experience of collecting and trying out new weapons. At least Air Raider got some proper love and is viable in singleplayer rather than totally dedicated support roles like in 4.1. For multiplayer in 5, look forward to bullshit health and damage scaling on enemies flat out multiplying enemy stats by the number of players in a lobby. Never played 6 but $60 is an absurd ask for the game and as a big fan of EDF, 5 did not impress me.
In terms of mainline games I guess there was also that mediocre game Insect Armageddon, but I don't really recommend it.