Look on the OPM sight under special salary rate tables and look for table 0558. This is the GS-12 and GS-13 with 30% additional pay scale. Remember that ALL of your ART pay (minus the obvious healthcare deductions, 401K, etc.) are taxed as opposed to your 10 year Major pay which has a significant tax advantage. ARTs do get matching 401K though. As far as "double dipping," it is really no different than any other job that gives you paid leave. I can work 40 hrs per week and get paid for 40 hrs, or take some sort of leave (comp/credit/mil) and get "paid" my civilian salary on top of my military pay. I am still working the time or taking leave - nothing is truly free (except the 15 days of paid military leave each year). Some other civilian jobs have similar options. Double dipping is a misnomer. In reality, it is not getting paid twice for the same work - that is called dual compensation, is illegal, and is perhaps one of the FASTEST ways to lose a government job and go to prison.
$450 for BCBS Federal is a little high (usually closer to $250-300), but that also depends on where you are in the country. Federal employees (including ARTs) are INELIGIBLE for Tricare Reserve Select (those who qualify for another gov't sponsored healthcare plan... yadda yadda)
Buying back time... Good deal anyway you slice it. Caveat - you cannot take an AD retirement AND buy that time back. You CAN take a RESERVE retirement (i.e. 30 years of service with around 17 or 18 years worth of "points") AND take a civilian retirement that gives you credit for those points, but you cannot receive an AD annuity AND a FERS annuity that credits you the AD time. Best part up front with no buyback required = you get "credit" for your AD time towards your leave accrual rate (4, 6 or 8 hours leave per two week pay period) depending on your total time, which can accumulate up to 26 days paid vacation (not including weekends or federal holidays) per year. You can usually stretch that into about 6 or 7 weeks off per year if you plan correctly.
Example 1 - AD person with 15 years AD time, and 15 years of civil service time = 30 years of civil service time with buyback program
Example 2 - AD person with 25 years AD time, and 15 years of civil service time = AD annuity (immediately) and 15 year CS annuity upon reaching an eligible retirement age (depends on several factors - see OPM FERS retirement pamphlet for more info)
Example 3 - ARC person with 25 years ANG/AFRC time (15 years of AD "points") and 15 years of civil service = AFRC/ANG annuity (age 60), 25 years based on IDT and AD time combined (15 years of points - new math, squirrelly, I know) + 30 year CS retirement with buyback.
Bottom line = you cannot qualify for a full AD annuity/retirement AND count that time towards a simultaneous CS retirement. I think there is a way to convert your AD retirement to a Civil Service retirement by waiving the AD annuity, and thus being able to buy back all the AD time into CS time, but you'd have to contact the experts for that one. Here is a site with some good info. Adding an ARC retirement to a CS retirement is definitely the way to go if you are not really close to 20.
The FERS retirement also assumes different criteria. For example, AD gets roughly 2.5% per year of service as an annuity, immediately. Civil Service gets 1%, 1.4%, or 1.7% (depending on career field and length of time in that career field) per year + matching TSP. 20 years AD = 50% base pay; 20 years federal Law Enforcement = roughly 34% base pay + TSP + SS; 30 years ART = roughly 34% base pay + TSP and Social Security at the appropriate ages. Hard to beat an AD annuity if you can get one. Do the math.
ART is a good gig as far as civilian employment goes, but as you know everything has a cost/benefit.