We have pretty low power rates in Idaho. I have a heater like that, 240V, 7500W, in my well insulated shop (foam R42 walls, R49 ceiling R18 sectional doors). I used it last winter to heat 1500 sq ft.
I have an energy monitor on the heater and set the shop at 50 degrees last winter. It was one of the mildest winters we've ever had.
Running the heater only during off peak hours, it consumed 57.8kWh on the highest day or about $2.89 of energy charges at those off peak rates. The highest month was 2230kWh for the month of January or $111.50. (just under $0.05/kWh off peak winter rate for energy charges. Delivery charges are a flat rate of a little under $30/month, often offset by Columbia River credits.)
I already had the mini split for the shop and am finally getting it installed, but didn't have time to install it before last winter and the resistance heater was $65 on sale at Northern Tool so I bought one and hooked it up.
It'll be 2-3X cheaper to run the mini split, but I'll never break even on the savings of running the mini vs the purchase cost. It'll be quieter and if our winters go back to normal, it'll have more heating capacity at a lower running cost and AC if I ever want it, but it won't last long enough to save me any money when heating to 50 degrees.