Ubercart

I don't see why not. I'd probably just keep it simple, unless you're willing to spend some more time on it. No need for AJAX. Add a textarea to product nodes for the prices in a minqty: price format, eg.

This also allows convenient pasting from an email, for example. Maybe even add logic to check if the price contains % and calculate the value (don't forget to round).

Munge the string and validate any parse errors at input. Convert it to an array and serialize it. Create a database table to save this data into, then load it back when the product node loads. Then in hook_cart_item(), inspect what you loaded as $item->price_tiers or whatever you name it.

I know the textarea thing sounds ugly, but for an admin only feature I've used the concept before and it works quite well.

(BTW, if you're willing to pay a bounty, you may convince me to write this myself...

I keep forgetting that lots of people are in UC 1 land. I'm working solely with UC 2 on a new site, so I can't answer for UC1. Look in the UC module directory/docs/hooks.php. Or, install the api module, which makes reading code quite nice.

As for where to put it, I'm guessing you're not a module developer. In my example above, I created a module called uc_flarg and it has a function named uc_flarg_cart_item(), which is an "implementation of hook_cart_item()". Read the developer docs for more information. (Also, note that modifying code in docs/hooks.php will do nothing. It's only there as documentation.)