Lot discount $6 each or 5 or $25

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Sorry if this question has been answered elsewhere. I couldn't find an answer searching.

I want to give a discount for buying a certain number of an item. Each item beyond that number would be at regular price until a multiple of the magic number is reached again. For example:

1 item $6
2 items $12
...
4 items $24
5 items $25
6 items $31
...
10 items $50
11 items $56
etc.

Is there an existing contribution that does this? If not, is there any interest in this sort of thing? If so, I can try to put it together to post as a contrib. Which leads to the big question... Any advice on doing this?  :D 

Thanks,

Paul