I have a post on my ID engagement experience this year. Can take it as reference.
If want to do customised built-in , pay for design and look for own contractor for full control if you have time .

The capetnery has different price range too if you use melamine or plywood etc. If you're not particular with plywood , IKEA is better .

Honestly if u want to avoid unwanted costs and overhead (eg: marked up pricing, management fees etc), you can skip IDs and come out with your own design, then once you have the rendering you can look for your own contractors. The materials and fittings eg: paint, lighting and switches you can bulk purchase yourself to save up even more

The software (Coohom or 酷家乐) takes some time to learn  and will cost a bit of money for high res renders or premium models, but its still much cheaper than engaging an ID. Yes takes time to learn and you need to spare some time also for researching and managing your contractors (essentially you are your own project manager) but if you really want to save money then that's the trade off...