Which is cheapest, Seachem Prime, Alpha or Safe?

Sometimes in life the problem isn't too few choices, it's too many.  I started keeping fish within this last year and I'm always looking for the best and the cheapest way to do anything.  I had settled on Seachem Prime to treat my tapwater for my freshwater tank.  I've been using Prime for months and loving it.  I'm about to replace my first bottle and I discovered that Seachem has two alternatives to Prime, Alpha which is sold mostly for saltwater tanks (and seems to be a more concentrated Prime) and Safe- which is Prime super-concentrated in a dry powder form.

Based on the reviews I've read and how much Prime does (removes chlorine, chloramines, makes ammonia safe, reduces the impact of nitrites and nitrates and improves slime coat without adding sugars to the water), I have been very comfortable with Prime.  It's so concentrated it turns out to be way more economical than the treatments that appear to be cheaper because they are cheaper per ounce.  A 500 ml bottle (about 17 ounces) treats up to 5,000 gallons of water.  I thought my bottle would last me for years (that is until I started adding tanks to my house).  

So, if Prime, Safe and Alpha all effectively neutralize all the bad stuff in my water, the choice comes down to price and convenience.  It seems Safe is no more difficult to use as it doesn't have to be pre-mixed, just added to the water as I do my water changes.  I always treat for the full volume of the tank, even when I'm doing a partial change.  I ran some quick numbers based on the prices of each product on Amazon.com in December 2012.  They are pretty heavily discounted on Amazon and all are extremely cost effective costing well less than a penny a gallon (the most is about 0.30 CENTS per gallon) at recommended doses. 

So, which is the least expensive (cheapest), Seachem Prime, Safe or Alpha?  I based my numbers on the size I was most likely to purchase. 

First Prime- a 500 ml bottle sells for $14.62 right now.  It'll treat 5,000 gallons.  Price per gallon is 0.29 cents (not dollars).  This time around I'd probably buy the 2L bottle, making it 0.17 cents per gallon.  The two liter bottle of Prime is $34.96 and would treat 20,000 gallons

Second- Alpha- a 1 L bottle of Alpha is selling for $25.42.  Alpha treats 50% more water than the same volume of Prime.  The recommended dosage of Prime is 5ml/50 gallons.  So, this bottle will treat 15,000 gallons.  Price per gallon 0.17 cents.  About the same price as the 2L bottle of Prime.

Third Seachem Safe- according to a Q&A on Seachem's website normal dosage is around 4 measures per 20 gallons with a a measure being 100 mg (it comes with scoop that is a "measure") .  The bottle on Amazon.com I looked at is 8.8 ounces or 250g.  It sells for $14.77  That'll treat 12,500 gallons for a cost of 0.12 cents per gallon.  

All three, Seachem Prime, Seachem Alpha and Seachem Safe would seem to be just as effective and easy to use.  In fact, I'm wondering if Safe might not be a little easier to use.  I started using a 5 ml syringe to draw my Prime out of the bottle. But, as the bottle got low, I used the cap as a 5 ml measure and I just estimate for my 10 gallon tank since the syringe won't reach far down into the bottle.  If I get the 2 L bottle, I won't be able to use the cap as a measure.  Safe comes with a dosage spoon and an 8.8 ounce jar would be a lot easier to handle and measure small amounts from than a 2 liter jug.

I'm going to try Safe.  It's not a huge investment, it's the cheapest and maybe the easiest and most accurate to use for small tanks (I have (2) 40s, a 10 and a 75 gallon tank).