I think I should go through the reasons for my opinion first before I say it.
Your art is fairly simple in that the characters aren’t complex nor are they posed in a complex way, and the edits are minimal. In addition, the effects are a bit overwhelming in some pieces, such as:

Here the grunge overlay goes OVER the character for some reason, which gives it the effect of looking like, for lack of better words, she has a skin disease. In addition, I understand that the character may be the way it is, but sometimes you have to make the artistic decision to modify some things (for example, in the case that the character is wearing multiple hats that are clipping, you have to choose the ones that are most essential and delete the others). In this case, this manifests as the head, which is comically small compared to the body. I’d recommend matching it with the r6 body rather than using an rthro head with it.

With this one, the aesthetic is nice but it’s very bright, making it hard to look at. As well as this, the effect you’ve added, which appears to be some sort of motion blur exclusively on the horizontal plane, is out of place with the piece, which is placed on an icey background. If you want to add effects, I would recommend something matching that, such as the following effect:
In this, the text is very hard to read because of the font, as well as the fact that it’s quite small. The background, which is stretched, also makes your watermark hard to read. I do like the colours in this one though, but I would suggest making that teddy on the shirt into an actual model (you can just use one from Roblox) to give it more form, since right now its just a texture plastered onto a block.
I think I’ve covered the main problems so far. For these reasons, I think you should price your work in the low hundreds of robux, like others have said.
Please don’t take this as discouragement or a reason to stop; I only say these things as a means of helping you improve so that you can price yourself higher once you do and such that your work has higher value in the eyes of other customers.
Hope this helps!