English Translation
It is not expensive; rather, it is affordable.
Why not sino? I have never heard of mas bien
I’d say, in this scenario they have a similar meaning, however, they don’t operate equally, and, in this case, “más bien” allows to keep a structure that is more similar to the english sentence.
- No es caro, sino accesible
You could also use “sino que” to allow the use the whole sentence
- No es caro, sino que es accesible.
I would have said that “más bien” was rather a translation of “but rather”; however having a look at the wiktionnary they seem to indicate it being the other way around, sino as a translation of (mainly) “but”, but including the meaning of “but rather”. While "más bien" as a translation of just “rather”.