Why does searching by contribution source use always the "Like" statement?
The reason is because Civi is trying to accommodate less tech-savvy folks than yourself :)

I recommend CiviReport - either Contribution Details or Contribution Summary reports.  That will let you control the operator, give you the total/average, and in the case of Contribution Summary, allow you to aggregate in other ways - e.g. total/average by month, or by financial type.