For each test case to be automated, code needs to be written, maintained and debugged.
The effort for automating all regression test cases is very high and the benefits limited.
Instead, use the following strategies for test automation:
1. automate the scenarios from the MONEY PATH; automate all scenarios that confirm that the users can pay for the products
2. automate scenarios for NEW FEATURES
3. add test scripts for NEW BUGS to be sure that these bugs will not appear again
4. focus on positive scenarios; there is limited value for automating negative scenarios
5. focus on functional scenarios; there is limited value on automating UI scenarios