Reword averages to guidelines in spending

Why is this change being made?

This changes the phrasing around what were previously the Average Price in USD, which were not always being treated as the guidelines they were intended to be, but instead as maximums.

This clarifies some of the language around these items to hopefully clear up misunderstandings and standardise how this page is interpreted across the company.

I originally floated this change in !48946 (comment 340372297)

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the change relates to any part of the project other than updates to content and/or data files (e.g. team.yml) please make sure to ping @gl-static-site-editor in a comment for a review and merge.
  • If the DRI for the page/s being updated isn't immediate clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies

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/cc @pkaldis, @daleb04, @m_gill