I wouldn't call this game quite done yet. It's a fun prototype, and it's a good idea. You can execute it a lot better, though.
Since you asked for feedback, I'm going to list everything. You already know some of this stuff.
* Graphics-wise this needs an overhaul. Put some sprites on it, make a background, and change the font. It doesn't have to be great art. I'm sure you can make some meteors.
* Center the text on the metoers. Seriously. TextBounds in LibGDX is your new very best friend.
* Actually, don't even use numbers for the metoers. Numbers are dull. Show the player how long till the meteor explodes in a different way. Perhaps, the color of the asteroid, or a progressive breaking-texture, like how Minecraft displays this kind of progress?
* Consider not scaling everything completely linearly. Towards an upgrade cost of 2000, the game becomes very easy. Too easy.
* Consider other upgrades than adding more guns (adding guns is fine, but adding 10 guns makes the game easy).
*** You can have slow or fast bullets.
*** You can have exploding bullets.
*** You can have bullets that can only hit certain types of meteors.
*** You can have guns shoot in different patterns, or upgrade one gun to spray, or burst like a shotgun.
* Any of the above could be power-ups.
* If I could control the gun (left and right movement), I could pick up things dropped by the meteors (powerups, coins, shields, extra lifes)
* Sound is always nice, in almost any game. If the user doesn't appriciate it, they can turn it off. Make sure the user can actually turn it off easily, though, and that the sounds don't linger on the system. Bfxr can generate nice sound FX, and there are tons of free tracks for music.
* How about a menu, or even just a title-screen?
* If you make the game more challenging, to the point where people can't go on forever, highscores become an option.
* It would be fun if large meteors would fall into pieces when shot. That would add some challenge.
* I think you should avoid ever giving the player the opportunity to spray the entire screen.
*** This can be done by not giving the player direct control over each added gun, or making the added guns much weaker.
*** You could give the added guns a dumb, inaccurate AI that would help a little, but not nearly as much as the player.
*** You could force the added guns to pause shooting while they turn towards a new enemy.
*** You could make the added guns have a much longer interval between shots.
* I think the field for the meteors is too small - especially since the meteors get so big so quickly.
* The game needs a death-condition. How about a game-over-screen?
* I think you should segregate score from currency for updates. That will make scores comparable.
* I think you should reward the player with points and/or currency only when a meteor has actually been shot down. Not before. I disagree with the notion of shooting 10 meteors a little bit, yielding the same points/currency as shooting one till it explodes.
* I just mentioned exploding meteors. That would be fun. Sounds too. I would feel way better shooting these meteors. Add some particles. Make it look fancy. Everybody likes a nice explosion. Make sure not to waste your already quite spare screen-estate.
* There could be some objects that the player should not hit. Like cows, paragliders, or airplanes.
Yes. It's too flat.