It sounds like what you really want is support for tiered pricing? I don't think attributes are a very good way to do that. Personally, I would write a small module which allowed you to enter multiple prices on the product edit form (or use CCK instead), then display these price tiers in a table on the product page instead of showing the Sell Price (do this by overriding theme_uc_product_sell_price, see http://www.ubercart.org/faq/617 for more info). Then, the Custom Price Calculation module could be used to set $item->price to the correct stored product price based on quantity. All price changes would be made in one place, on the product edit form, so this would be easy to maintain.

It's even easier if you have a formula you can use for the pricing that applies to all your products. For example, qty 1-9 full price, qty 10-19 5% off, qty 20-29 10% off, etc. - then instead of storing many prices for each product you can just use the formula in Custom Price Calculation and in the theme function.