I used to run a printing company, and did all of my invoices on the lowest quality paper i could - something like rapid recycled at 80gsm, or xerox bulk paper. The point for me was that if you are invoicing them they have already used your service - and they should know that your work is quality. Tesco do a box of 75gsm £12 for 2500 sheets (.5p per inv)
If you want better quality paper though I would suggest cream laid certificate paper (100gsm) around 8p per sheet (at currys for example), which look great when printed on...
Hope this helps,
David