This!
You can't be making any money at all, making a cake like that for £30. It's a lovely cake, but you must be making no more than a tenner, if that , once you take into account ingredients, box, board, electricity, gas, petrol, contribution to equipment, colourings, baking parchment, insurance etc. Then there's the time taken shopping, doing admin, taking the order, baking, decorating, clearing up etc. You won't even be close to half minimum wage.
That's fine if you just want to make cakes for friends and family, a hobbyist. But if you want to sell to the general public, you'll never make any money charging like that. The PP is correct, value your work. Otherwise it's not fair, either to yourself, or to other local businesses. I've seen many businesses charge like this, because they worry they're too expensive if they charge more, or don't work out their costs correctly, but they never last as a business, it's just not sustainable