This "product watch is for the item in the title. Obviously it may not be exactly what you use but it should give you an indication sites with good card prices.

As a start

Alan and Barrys Crafts have:
50 x A6 Deckle Edge Pastel Cards & White Envelopes @ £10.99 which work out at just over 20p for a card and envelope.

http://alanandbarryscrafts.com

(I know these have a deckle edge but comparisons can be plain or deckle)

OK it took a bit of working out but at Ideal Envelopes you can buy 100 white envelopes @ £4.70 and 100 white single fold cards for £9.19 giving a total of £13.89 or 14p for a card and envelope (hope I've worked that out correctly)

http://www.ideal-envelopes.co.uk

If you go to http://www.handyhippo.co.uk they do a bumper pack of 50 A6 white cards and envelopes for £4.95 which is just about 10p for the card and envelope.

If you go to http://www.argos.co.uk/ (yes that's right argos )
and have a look in their Art and crafts section
You will find a pack of 50 plain white A6 cards and envelopes for £4-97
Or a pack of 100 for £8-99 which is the better buy at just under 9p per card & envelope (If I did my maths right )

Being an argos store if you have one near you, you can pick them up without having to pay postage (an even better bargain then)
If you dont have one near you will have to pay postage

They are available in an argos "extra" store so if you just have a standard store near you you can reserve them online and if in stock will be delivered to your standard store within a couple of days for you to pick up
Or if you are having them posted will be as stated on their site

Crafting Direct have a Mega Deal but I can't work it out

100 Cards, 100 Envelopes and 100 Inserts £15 and now for the interesting bit "Buy sentimental and get everyday pack FREE"

Not 100% sure what it means by that but you might wanf to pop over and check it out.
CRAFTING DIRECT

http://www.dscrafts.co.uk/windmill_series.htm
A6 White 290gsm Card Blank and White Envelope £2.98