In so far as I know, modding the game this way is NOT supported by the devs, but it is possible, so either have fun or move on.

Yes, this is a cheat, get over it...
Basic mod, but no one seems to even talk about this, so here is my guide for how to do this.

Have fun...

Oh and thanks EXOR Studios for making this game THIS easy to mod.
LOVE IT!
Hope your new game (The Riftbreaker) is just as moddable!

I recommend the following 2 programs:

7Zip - https://www.7-zip.org/
Notepad++ - https://notepad-plus-plus.org/downloads/

I also recommended you learn the concept of how to backup files BEFORE you edit them ;)

I will, for the sake of shortening this guide down a bit, assume that you have little more then basic knowledge of navigating Windows folders, making backups, and opening/closing a .zip file.

Navigate to:
.. \ Steam \ steamapps \ common \ X-Morph Defense \ packs

Open file: 00_win32_data.zip

Navigate to:
.. \ scripts \ towers

Extract/copy towers.tower to a place you wish to use as the backup folder.
Now copy same file to the work folder and open that with Notepad++.

Divide the cost of the build_cost with 100 (so 300 will be just 3)
Do that with all the build_cost.

Do the same with upgrade_cost.

Now replace (drag&drop) the towers.tower file in the .zip with your modded version.

Voilá! You now made towers free!!!

NOTE: Don't play Multiplayer/Co-Op with this kind of modification unless your partner also got this modification, or you my experience severe instabilities...

If you want to restore the default settings, just replace (drag&drop) the backup towers.tower file in the .zip and done.

Now before i tell you how to, just need to let you know that this can also be done to most other towers. But all you really need is the Basic Tower, Flame Thrower and SAMs.

Just as before, Open file: 00_win32_data.zip

Navigate to:
.. \ entities \ units \ alien

Extract/copy tower_ground_basic.ent to a place you wish to use as the backup folder.
Now copy same file to the work folder and open that with Notepad++.

Scroll down to line 502 (About 7th of 8 way down.)
Locate range_max and change it to 32.
Change aiming_range's value to 37.

Scroll a bit further down to about line 549 and locate fire_rate
Change it's value to 0.075

Now just Drag&Drop the tower_ground_basic.ent you just worked with back into the .zip file from where you got it and enjoy super fast, long range basic turrets.

NOTE: Due to their crappy tracking and prediction abilities, they aren't the "end all, be all" turrets,
just great all round turrets.

This Mod just makes them more useful - If i find a way to make them shoot, for instance, homing bullets, then they will replace pretty much ANY tower :D