As others have pointed out, the 6D will not be going that cheap, probably not ever. Last-gen cameras just don't drop that much in price when the next gen comes out, and you are talking about a camera that was $2000 at launch.
I would keep an eye on the Canon Refurb store for the 6D; right now it is $1099, the same price as a new 80D (non-refurb) and a few hundred more than a refurb 80D.
All that said, what you absolutely need to do is pull together your "list of grievances" with your 100D. What makes you want to replace it? Is it low-light performance? Burst rate? Max shutter speed?
Also note that you probably have at least one lens with that 100D, and it will work okay on the 80D (in fact, it is likely the same kit lens that comes with the 80D, the 18-55 IS STM) but likely not at all on the 6D. Canon has two lens standards - the EF and the EF-S. The EF-S uses the same mount as the EF, so a camera which takes an EF-S lens will also gladly accept an EF lens. However, EF-S lenses "light up" a smaller circle, which covers all of a crop sensor but not all of a full-frame sensor. EF-S lenses, as a result, are often significantly lighter and cheaper for the same level of quality as an EF lens (although, since the "pro" series cameras are all full-frame, the best quality lenses are just EF mount). Thing is, the kit lens that came with your 100D will be EF-S, as most likely will be any lens that you bought along the way to go with it. You can sell those and get a little money back, but low-end lenses go for a small fraction on the used market (compared to, say, a high-end 'L' series lens which might sell used for 85% of the 'new' price or more; the $150 kit lens 18-55 sells for $25-30 used, and that's actually a fairly good return). So account for needing to get at least the kit lens with the 6D.
And, if you are already divesting of your lenses, you owe it to yourself to consider other brands as well. If you draw up that list of grievances, you might find that a mirrorless camera might actually serve your needs better, or maybe instead of going to full-frame you would do better off going to micro-4/3.For street photography, going smaller, rather than larger, is probably a better choice (fashion and portraits would push more towards a larger sensor though).
Not that I'm saying you should ditch Canon - I have been a very happy Canon shooter for a decade and change and absolutely prefer their cameras from an ergonomics perspective - but it would be silly to ignore that you are essentially "starting over" and so will never have a better time to reconsider brand/line than right now.
Above all else, and I apologize for beating a dead horse, you need to figure out why you want to buy a new camera, and then tailor your decision of which camera to buy based on how many of those real grievances the new camera will fix. Those on photo boards generally term the "it is just time to buy a new camera" urge "gear affectation syndrome", or "GAS". It is the source of many a bad and expensive buying decision. When you start from a decision, then start writing down the things that will improve with that decision, it is easy to rationalize anything. Start with what you want to improve, and how important each of those are, then determine how well that list matches each of your candidate options, including "just keep the camera I have for a few more years".