I am experienced with Blender and Unity and have created, edited and fixed VRChat avatars of all kinds for myself and many others.
On their own, basic avatar conversions will typically cost $15 and up, depending on the model. You'll end up with a unitypackage ready to upload, as well as the original files.
All finished avatars will be atlas baked and decimated less than 20k tris. For more complicated conversions, such as below, it may cost more. Please send me the model to begin with so that I can determine how difficult and whether it will be possible to convert it.
Regarding 3D assets and characters from a third party: you, the client, are responsible for attaining permission to use the asset or character from the third party.
Reconstruction and big changes can cost $30 and up. I'll work closely with you to ensure you get the look you're looking for.
For tough decimation jobs, I carefully decimate individual pieces to maximize quality. I'll prevent any holes from appearing in your model ingame, and prevent unwanted texture stretching and distortion. Proper decimation can take time. Very detailed models can cost $10-30.
For models that don't come with blendshapes I can create them from scratch. Basic lipsync (ah/oh/ch) and eye tracking (blinking and functional lowerlid) usually costs $10.
Let me know what you want - exaggerated cartoonish talking, like Shaggy (below), or realistic, subtle movement, like most mmd models come with.
Making a model from scratch or from a basemesh is going to cost quite a bit. However, the first 2 avatars pictured below are very simple models that can be done quickly and relatively cheaply, for around $50.
I try to minimize costs by using the human character generator ManuelBastioniLAB, which requires the basemesh to be credited (in your avatar portrait). The third avatar pictured below is an example of this.
Finally, I can model a detailed character from scratch, generally costing $150 and up.