Obtain submission price from variable

[sergei-maertens]

Closes #4764

Needs forward-porting due to new migrations.

Changes

[Describe the changes here]

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • [x] Checked copying a form
  • [x] Checked import/export of a form
  • [x] Config checks in the configuration overview admin page
  • [x] Problem detection in the admin email digest is handled

• Release management

  • [x] I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • [x] Ran ./bin/makemessages_js.sh
  • [x] Ran ./bin/compilemessages_js.sh

• Commit hygiene

  • [x] Commit messages refer to the relevant Github issue
  • [x] Commit messages explain the "why" of change, not the how

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.57%. Comparing base (72320c7) to head (10404b2). Report is 10 commits behind head on stable/2.8.x.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## stable/2.8.x #4770 +/- ## ============================================= Coverage 96.56% 96.57% ============================================= Files 746 746 Lines 25186 25229 +43 Branches 3315 3320 +5 ============================================= + Hits 24322 24365 +43 Misses 601 601 Partials 263 263 ```

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