Pricing Increments

I have a client that wants to sell raffle tickets using a per dollar amount model instead of per ticket model. For example, raffle tickets are $1/each and you need to purchase a minimum of $10. I’ve got that part easily taken care of with a minimum quantity plugin but now they want to add a wrinkle. For every $5 spent you get 6 tickets instead of 5 and the same works for every increment of $5. So if they spend $10, they get 12 tickets instead of 10 and so on. I’m not sure how to implement this pricing model. Does anyone have a plugin they can recommend?