I've been talking to Aegir lately, and we both think this is a good idea.

As you complete campaign, you can unlock up to 10 digging sites. Using these digging sites, you can find old workshops and discover items. When you discover an item, you have a more likely chance of getting that item in a box. However, if you're lucky, you will discover a mythical item. Until you discover a mythical item, you are unable to get any mythical items in a box. When you discover a mythical, you can now see that item in the mythical vault. You can upgrade digging sites to either increase discovery rate, or increase the chances of finding a rare item. But be careful! These digging sites cost money to run (5,500 credits every week). If you do not pay the fee for running the digging site, that site will be abandoned, and you can no longer use it. A warning will pop up when you need to pay the fee. As you upgrade digging sites, the fees will become more and more expensive, so be careful. If all of your digging sites are abandoned, then you can reset your discovery progress, meaning that all discovered items you currently have will once again be unknown. All mythicals you currently have will be kept.

I like the idea. Maybe the cost increasing is just too much

What do you mean by this? The cost only increases when you upgrade a digging site. Seems reasonable to me.

The good ideas keep on coming. :3 What we need now is the devs to implement this. I mean, Tactisoft can still make a profit even with this, no harm done adding this in. It's a win-win strategy.