Question about database structure and eCart discounts

Our client has a basic list of plant products as follows:

productID
productname
productprice

Our client now wants customers to be able to purchase these products in three different categories:

3 Plugs - £1.15 each
9cm Pots - £1.75 each
1 Litre Pots - £3.50 each

However, if a customer purchases large quantities of any category then a discount is to be applied and this discount needs to be different for each category. For example:

15 x Plugs = £15.00 (£1.00 each)
30 x Plugs = £29.50 (£0.98 each)

15 x 9cm Pots = £24.00 (£1.60 each)
30 x 9cm Pots = £44.32 (£1.54 each)

...and so on.

I just wanted to ask advice on the best approach to this. Should this pricing be stored in a database table and be referenced by, and calculated on, the product detail page or should I use discounts/charges in eCart?

It struck me that configuring it all in eCart could be quite a task, considering the complexity of the calculations.

I'd appreciate your advice. Thank you.

Regards
Nath