What does annoy me is that you can buy a perfectly good running t shirt in a Lycra type material for about a tenner but make it a bit longer at the back and add 3 pockets and suddenly it is fifty quid
A running top is a very simple raglan sleeve garment with a hemmed neckline. a £50 cycling top has 3 pockets, often one pocket has a zip, a collar, usually a zip at the neck if not all the way. It's also made of fabric with a high lycra content, and stretch fabric is more of a challenge to sew. Running tops tend to be made from a drapey rather than stretchy fabric. There is a ton more engineering in a cycle top compared to what is basically a t-shirt, so it's going to cost more.
(I once made a cycling jersey, just to see. It was a faff. Also, I wanted to make it unique - no point sewing something at home you could buy in a shop, so I made it with gold and black fabric. I looked like a Star Trek cosplayer).
You could always wear a running top cycling.