Give a positive gas cost to the BALANCE instruction

Context

This MR fixes a small bug on the gas cost of the BALANCE instruction. It assigns it the same cost as all other step constants (10 milligas) instead of 0. The bug seems to be a consequence of !3871 (merged).

TODO: add a benchmark for the BALANCE instruction (there is already one)

Manually testing the MR

Check the regression trace diff; the cost of executing BALANCE should go from 0 to 10 milligas.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
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