How to correctly evaluate the development of the project?

Question

Project management

Hey! I work as a freelancer, I am engaged in the development of online stores and landing pages. I carry out the whole cycle: from writing technical specifications, to design-layout-programming and content filling. And now I realized that there are specific miscalculations with the cost of work. When evaluating the cost of a project, I always start from the average market values. And if I can calculate the drawing of a design or filling it with content quite accurately, then there are problems with programming. Almost always, there are pitfalls somewhere, which in the end can take up to an additional week of working time. As a result, projects are stretched out in time, which causes dissatisfaction with customers. I tried to multiply this stage by 1.5 or 2 times, but then the price is absolutely indecent.

Answer 1

let's always plug, let's say it will take 1-2 days and for the money 100-200 bucks

Answer 2

a) Work agile (T&M)
or
b) Find a good project management course and learn at least the following topics:
All bicycles in UE have long been invented. It is not productive to try to reinvent them by collecting a few ideas and parts from other people's bikes.