It depends where you are of course but the usual cheapest gaming pc is an older pre-built, like a Dell, that you then upgrade with a videocard. The total cost will be the price of that computer plus the cost of the videocard. But you will have everything you need. The downside is you have to be careful in your choices. You want a mini tower, not a small form factor(SFF) or desktop(DT)case. You want to make sure it uses a standard power supply so if you have to replace it you can buy a new one. You want to make sure the motherboard uses standard power supply connectors, otherwise even if a normal power supply fits you still might not be able to use it.
If you think you're going to play Fortnite with a GT 710 I think you shouldn't buy anything until you do a little more research about what game system requirements mean. You can't just pick the cheapest of anything and think you can run games on it, it doesn't work like that. With PC gaming, the individual games all have their own requirements. Some games require a lot, some games don't. Since you know what games you want to play, look into what kind of components it takes to run them.