Voiceover Pricing set-up

Hello there, newbie here attempton to build my first site.
I’m wondering about the best way to do pricing and whether there is a suitable plug-in etc.
My site will be offering voiceovers and I intend to offer three levels (seperate products) of voiceover:
Small: A base cost of ￡5 for upto 100 words and then an additional ￡5 for each further 100 words
Medium: A base cost of ￡80 for upto 1,650 words and then an additional ￡5 for each further 150 words
Large: A base cost of ￡160 for upto 4,100 words and then an additional ￡5 for each further 200 words

I also want to be able to offer a discount of 20% to customers who have put quite a lot of work my way.

I’m afraid I have no idea about how best to go about this, so any advice would be appreciated.

Cheers,
Alasdair