It is important as an artist and shoe customiser that you understand what you're worth (at a specific time of your career) and I'm going to give you some tips on how to price each piece from the experience I have had in the industry. This is my own opinion and you can choose whatever you want to do with it, these are tips I think are working well for me and will do in the future.
With my shoes prices, I know from lots of experience the worth of my art based on many factors, including the time, the skills I have acquired over time, the shoe pric and whether it is a 1 of 1 request or not.
One of the best ways to understand the pricing for your customs is to test the terrain, meaning make as many designs as you can, whether that digitally on a drawing programme that you can show the customer or physically on shoes. What I mean by this is to upload these designs onto your social media or website showing a set price and just see if any sell at all and then you can pick out the most popular.
For me, I used to make a design on a piece of paper (before I could afford stuff like iPads etc) and message hundreds of people a day for the possibility of someone seeing it and wanting it. I would do a guesstimate of the price that I think is correct based on how much area the painting covers.
Have a price range
Once you have a few designs that are different and popular, you can set a price range, for example, if a pair with just the swoosh is painted this will be a lot less than a pair with the whole shoe painted. This could be around £150 for the swooshes (inc the shoes) and more than double that price for the whole pair of shoes painted, but this price totally depends on how much you think your artwork is worth.
Some customisers that mainly work with 1 of 1 pairs will start their customs around £300 for example because they know how long it will take them and they know it is unique and only one person will have it in the world, so they need to include that in the price. Whereas, more common designs like trending brands (Louis Vuitton, Bape, Gucci etc) will be a bit cheaper because of the increased saturation in that market so between £100-250 for example.
You need to be consistent with producing art that you can showcase on social media, whether that's stuff you're selling or just doing for fun, and overtime you will have hundred of different art pieces that you have painted. And eventually overtime, the time scales you paint within, your brain will work out an estimate for how long each piece will take. This is very helpful because then you can give yourself a set rate per hour and get a specific price for each piece.
With time, your designs will get better and better as you practice more, and this part is great as you know you are getting better over time so you can start increasing your prices to help calm down the amount of pairs you paint yet still have a good income for yourself. This is great for you, but also great for the customer to know that your designs have improved and you can maintain great quality work for unique pieces of art that you know are worth the price.
If in doubt, work for free
Everyone says "DO NOT WORK FOR FREE", I personally think this is stupid, It totally depends on where you are in your art venture and what you're doing for free. At the start of my custom shoe business, I would paint people shoes for free just to post on social media in the hopes of getting attention from future customisers, this is a great way to show your passion for art that you're willing to work for free knowing you will get more attention.
Another reason you should work for free (and I still do this now) is when you're painting something that has the potential to get lots of attention from a popular social media star or brand etc. In my opinion, It is best when you actually know the brand or are a fan of the music artist for example and then you will have that boost of motivation you need to order a pair and paint it knowing it will benefit not only you but them also.