perf: optimize update_purchase_cost method (backport #38298)

[mergify]

This is an automatic backport of pull request #38298 done by Mergify.
Cherry-pick of bcbe6c4 has failed:

On branch mergify/bp/version-14-hotfix/pr-38298
Your branch is ahead of 'origin/version-14-hotfix' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit bcbe6c4a53.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   erpnext/projects/doctype/project/project.js

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   erpnext/projects/doctype/project/project.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[frappe-pr-bot]

🎉 This PR is included in version 14.50.0 🎉

The release is available on GitHub release

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