The problem with extension cords is thin wires + long distance. What you are describing sounds totally fine, because 10awg wire is huge and 10ft distance is very short. According to the tables you can run a 15amp circuit on 14awg wire, and normal 20amp home circuits are 12awg which is thinner than 10awg.
https://www.cerrowire.com/products/resources/tables-calculators/ampacity-charts/I have never personally seen a 10awg extension cord, but adding a 10ft extension when even using a cheapo 16+awg extension cord is common. You'll notice that beefy electrical appliances may have flimsy power cords because it is only a short run.
You'll be fine. Just don't let it fall over or set anything nearby on fire.
As far as cost, I used to run two of these 15amp space heaters in my old attic greenhouse. It probably added $150 or more to my power bill but it wasn't a killer. My power cost around 11cents/kwh. I also had a propane space heater as a backup in case of power outage or
really cold nights where the electric heaters couldn't keep it warm enough
And one last thing, get a wireless battery powered temperature alarm in case you lose power while you are sleeping. They have saved me many times.