Hey, I was looking up miniatures and it seems that heroforge.com makes you pay to get the files for 3D printing. Is there any website where you can just design a miniature and print them for free? Thanks! (Also It would be nice to find a place to design Large to Gargantuan Miniatures...)

There’s no place that does it free that I know of. 
For large minis, there’s titan forge. And it might also be closer to what you’re looking for.
I think it’s actually better if you do a lot of printing. Where in heroforge, you buy the one mini, with titanforge, you buy (a license to use) the component parts. Then you can re-use those parts mix and match style to make as many minis as you like. Download the file and print it yourself. Swap things around, change the pose and download it again, no extra charge. That’s my understanding, at least. 
They also do print for you if you need. I don’t have a printer, and got a size large mini printed by them for my rune knight for when he embiggens.

I have downloaded and printed probably 100 mini's from Thingiverse and Cults 3d.  They aren't editable, but they work pretty well for me.  I have a cheap 3D printer and it does ok with them.  Not the professional quality form heroforge by any stretch, but certainly functional.

 Is there any website where you can just design a miniature and print them for free?

Not that I have heard of, and anyplace that did do it would go out of business quickly without some sort of decent revenue stream to support it.

aside form the cost of printers and materials, there is also the shipping aspect to consider, and the paying programmers to create the design stuff and modelers to create the models. All in all, if someone is getting a 30mm fig for less than 10 bucks they are getting a bargain.

The Print for free part you could probably get away with through a friend or a friend of a friend (if you can trust them with your fig) -- it is the design part that really gets you.

I had a good bonus earlier this year and was able to pick up all of the Titancraft packs they had at the time. I tend to prefer their overall way of doing things to the HeroForge approach, but I also gotta say that I sometimes like some of the options on Heroforge way more than those on Titancraft and vice versa.

I use it to print five fifigs for each of my players, and shipping is hell on my scraping by as it is budget. 

It's not designing your own, but someone has created for free all of the monsters in the core books;

3D Printing Dungeons and Dragons with Miguel Zavala - 3DPrint.com | The Voice of 3D Printing / Additive Manufacturing

cool

First of all I'm going to repea Tlawrence84 in that Miguel Zavala has made free STLs of all the monsters in the core books available for free on several different sites.

For PCs and NPCs I've been using titancraft. They have a bunch of free placements you can use but also have some assets you can purchase and reuse as many times as you'd like. 

This was great for when I'm planning a large encounter I can make a bunch of different NPCs that I can print for free, or getting a specific pack or item that I need and using it a bunch  of different ways. I think the level of customization is a little less than Hero Forge but the cost in the long run is a fraction.