It’s an easy question to ask “How much does it cost to start a blog in 2021?”, but it’s a devilishly difficult question to answer.
I could counter with, “As much as you want to spend”. It’s a bit of a cop-out but the truth is what you spend starting your blog will depend on your budget.
The law of diminishing returns definitely applies to blogging. At the end of the day, however much you spend on tools or gizmos, expensive themes, or first-class hosting, it will be worth diddly-squat, zilch, nada if you don’t produce content people value.