Wow - thanks again... (FYI: your link is missing the s at the end of cost (..\cost
s.htm - I used
this)
And yes, cost is really screwed

This is what I ran into when designing for a contest.
Your curves tell the story - the design rules come from simple curves - with irrational disjoints when put in tables (possibly simply mistakes due to the added editing effort). An additional issue is the silly system of giving a letter code - totally inconsistent with most other things which used hex - instead of just stating the rating for the given hull (1-6 or above - simple and also fits hex nicely if one wants to pursue that).
Look at the starship examples - they vary by ship and by books as to whether the drives are 'coded' or simply rating is stated. It should be obvious the codes are worthless (by themselves they require a table to lookup and other tables to be useful in design). One can also see the difficulties with the use of AA-DD and the capital ship codes.
Further, as far as I can tell, MGT dropped ship profile coding, so this is meaningless - just state tonnage. Look at the tables in HG - they state tonnage in most cases, or a code range. The code range requires looking at the code table - what an absolute waste and added burden. Not to mention, loss of granularity at the higher end (to cram numbers into an artificial alphabetic encoding).
Given this is a roleplaying game - and not a RW technical simulation system - the small craft tables really look silly. Again, a simple set of formulas would have done a much better job (with tables for common cases if desired - but lets face it when CT was first out calculators where a luxury few had).
P.S. - I don't generally like to compare to CT, since there are so many differences (For capital ships, MGT M-Drive tonnage is
way down from CT, while J-Drive is identical; Jump drives are half as expensive, while M-Drives are the same for G-3 and above; but HG ships start at >5,000 tons).