I want to improve my math skills, now I'm trying to find good route for this purpose. I'm using Khan Academy, and recently found "CS103 - Mathematical Foundations of Computing" course which looks good but pretty expensive for me.
Is some book or not expensive/free course available which gives a good foundation in math for future usage in different fields such as statistics, algorithms, data analysis, machine learning etc?
If you are asking for a book on Math Foundations in CS, it is my opinion that such a book would emphasize logic, set theory, counting principles, functions and relations, and other discrete structures such as lattices and (especially) finite state machines. The analysis of algorithms would probably come next in this list, but certainly not statistics, data analysis, or machine learning as "foundations" topics.
Personally, I think Book of Proof by Hammack or How to Prove It: A Structured Approach by Velleman would be good choices. Another good book would be An Introduction to Mathematical Reasoning by Eccles.
I might also recommend Concrete Mathematics: A Foundation for Computer Science by Graham, Knuth, and Patashnik. This book has been at Stanford for many years as the text for their mathematical foundations for CS course and is quite popular these days.