It is not normally recommended to share clients. Coloring for extensions and what looks natural is not always the same as what is needed for extensions. It will cost you more time and effort if you are not on the same page.
If you are going to share a client, then you do NOT have to alter or change your prices. All new install and refresh prices are PACKAGE pricing, including color on the client and the extensions.
You will still have to color the extensions, tone, smudge, etc., to ensure a seamless blend.
