[#514] Fix RPC-based fees calculation

Description

Previously I didn't test fee evaluation properly and thus we now have problems.

This MR tries to fix the issues.

Tested:

• For all our scenarios against a public node (see these jobs)
• Check that the original issue in the other repo is resolved - if this is going to be checked soon.

Related issue(s)

Resolves #514 (closed)

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• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

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    • Haddock
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  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

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