jUnit to implementation ratio in a Continuous integration project

in a new big project we are doing jUnit and continuous Deployment to production for the first time. jUnit plays the role of unit testing only, for integration testing we are doing selenium tests for frontend testing separately.

We see that it takes an unexpected high amount of time for testing all relevant test cases. Testing sometimes takes 60-70% of the implementation effort.

Do you guys have some experience values for this ratio? What ratio can be considered still reasonable?

We know that it is the best to aim 100% test coverage and coverage of all relevant test cases - however having a test effort of 60% seems to blow up the budget of the project.