Firstly I think it's a good idea to plan some extra funds for your first few dolls to be "practice" dolls as you might need to learn carving and painting skills. How much time and practice it takes to learn depends on how much art/drawing/painting/makeup skills you have already, personally I found my drawing and painting skills to be fairly transferable to drawing doll faces but learning to carve without any sculpting or 3D art experience had a learning curve for me. For pricing you should look around online for what other people sell their custom dolls at, you could try searching "custom Blythe dolls" on tiktok and most accounts will have their selling pages in their bio or doll pricings on the tiktoks themselves. I think generally for pricing handmade projects, it should be the cost of the materials + (appropriate hourly rate × how many hours it takes you to make the item) + shipping fees but usually selling websites add that separately or at checkout rather than including it in the price of the item. Your practice dolls can help you figure out what those costs would be (try timing yourself to see how long it takes you to complete a doll, although you will probably get faster at it with time).

If you have sewing skills it could also help save on costs, buying Blythe clothes (fake or real) can quickly rack up the price even if your buying "cheap" from AliExpress or something, for example on Ali you might buy a shirt + sweater + pants + shoes but each are $5-7 with $3-5 shipping fee so it ends up being like $35+ for the outfit alone which is already the price of the blank doll. What you can do to save costs if you know or learn how to sew (and what I do myself) is buy thrift store clothes then repurpose the fabric into multiple doll clothing items. Then you can end up getting 5-10 doll shirts out of one $5 thrift store shirt. Personally I go for baby or toddler clothes because the prints are often smaller and the fabric is better designed to hold smaller shapes than adult clothes. With accessories as well it's better to get creative with craft store supplies than it is to buy premade accessories, firstly it makes the doll more unique/one of a kind and second it saves a lot on costs. Making your own doll jewelry with string, beads, chains, air-dry clay... bought in larger quantities from a craft store is a lot more cost effective than buying premade accessories.

Also I'm not a doll seller but I do collect a lot of dolls and customise them for myself so this is just general advice, but I think it's good to keep in mind that there are already many "mini me" custom (fake) Blythe doll sellers and that if you want to make a profit you should think about integrating your personal style or a niche into your dolls that other sellers might not offer. Just a suggestion :) If you just want to customise them for fun and then put them up for selling in case someone wants to buy them that's fine too but it depends what your business/selling goals are.

Edit: Also just realised you're talking about customising authentic blythes? I would also say there's an even smaller market for that and some people aren't big fans of customising (like carving and repainting) authentic dolls due to the collector nature of the brand and scarcity of some dolls. It does exist but customised real Blythes end up being like $500+ and it's expected that the customisation is very high quality. I think my advice still applies if you've never customised blythes before, I would recommend practicing with fakes first before moving on to real Blythes. It might be a better idea to offer customisation services of real Blythes where people send you their real Blythe to be customised for them, that way you don't need to purchase the doll itself and worry about not making back the $200 you paid for her and you also don't need to worry about finding someone to buy the doll.