You’re just paying for support in case you need it, which after 6 months you probably won’t be needing.
As for an arm and a leg, the prices are not expensive, for $50 ( generally what a theme costs ) you’d get less than 1 hour of a developer’s time. If a theme is an arm and a leg then having a custom website made would be “your whole body, mind, soul and the same for your offspring and their offspring and so on for 500 years”
Important to note that most theme providers have a yearly subscription and once that passes you can’t even get updates. On themeforest you always get access to updates, the subscription is just for support, in case you need it after 6 months, which 99% customers don’t.