Add cost model enum

[chris-ashe]

Description

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 48.67%. Comparing base (808481e) to head (2771a30).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
process/main.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4469      +/-   ##
==========================================
+ Coverage   48.64%   48.67%   +0.03%     
==========================================
  Files         151      151              
  Lines       29704    29729      +25     
==========================================
+ Hits        14449    14471      +22     
- Misses      15255    15258       +3     

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