You are correct, I hadn't properly done the math.
For a 500 gram animal 60g prey is fine. You seem to be at the right prey weight. However it is going to start costing you considerably more to feed them as they grow if you cannot find a supply of rats.
Imagine, your snake has reached 1000g and by following the 15% prey/body weight ratio. You would have to feed five 30 gram mice every seven days versus a single medium rat in the 150g range.
Where I live, if I am buying Frozen prey, a rat that size is $3.99 and the mice would be $1.59 each. So you would be looking at a cost of $4 versus almost $8 a week.
I personally have come across a local rodent breeder who gives excellent prices and my feeding cost has dropped exponentionally.
Before I found my new supplier it cost me over $100 a week to feed my collection and they were all on frozen rats. Now I spend about $150 a month. It would benefit you greatly to find a local supply of food for your animals prior to adding to your collection.
So in answer to your original question. No, you are not feeding too much IMO. However you could probably cut back to every 7 days at this ooint and still see them growing like weeds.
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