It isn't, stable diffusion + anything v3 or protogen is free
Midjourney costs 10usd/month for iirc nearly 2 hours of gpu time and is better than dalle
Looking at what others are doing is very helpful.
This prompt book has also been the best reference for me, even though it’s not specific to MidJourney: https://openart.ai/promptbook
other tips: add on the end --ar 3:2 for wide image or --ar 2:3 for tall
most important part of prompt toward front, usually what kind of medium the art should be (coloring book page, stained glass, watercolor, etc), then describe the content and use photographer terms like portrait, landscape, or long shot to describe framing and what's in the image, things like background and foreground help too.
if you know much art or photography theory, it will give you superpowers over normal users.
take some time to look at other people's prompts on lexica and midjourney showcase, if you want to play with stable diffusion for free first I recommend playgroundai.com which only requires a google account for about 300-800 free spins a day (they throttle you a bit after 300), but I think it's a good way for people to dip their toes in the water, but midjourney does tend to give better default results
So you're paying $15 to get 11 great images that are what you want? That still seems pretty cheap.
The point is, the cost includes the failed images. If you wanted the cost to only be for the good images, and you get free bad images, they'd simply charge 10x the amount.