Once your card is complete, you just have to print it. Suggestions here include not using any duplex function your printer may have, even though it may appear quicker. Duplexers nearly always involve tightly curved paper paths and using card stock may be difficult for a duplex printer to handle, resulting in paper jams and damaged cards.
Instead, try to arrange the most straightforward paper path you can - some printers may have an occasional feed tray especially for this - and print each side separately. It may take a bit longer, but you'll end up with flatter, neater cards and less trouble with the print run.
To print the back side of the card, select File, Print and type in 1,4 as the pages to print (put a comma between the numbers, not a hyphen). To print the inside pages, take the card from the output tray of the printer and reload it into the input tray. Repeat the print process, but enter 2,3 as the page numbers.
Knowing which way up to insert the card when printing the second side, can be tricky, but a simple way to check and not waste a half-printed card is to take any sheet of paper already printed on one side, place it in the feed tray, note which way up it is and print the second side. If the new printing comes out on the clean side of the page, you put the test sheet in the right way up, if not, put your cards in the opposite way up from the test page.
Costing out the printing of the cards is the price of the blank card - with envelopes - and the ink to print it. Card and envelopes for, say, 50 cards should cost under £10, but if you're front page design has any quantity of colour in it, you should assume you're going to print 50, 15 x 10cm photographs; the ink use will be similar. In most cases, if you allow one set of cartridges for your cards, you should have some ink left over at the end.
Even so, in the cheapest scenario, with a simple black and tricolour cartridge, you're looking at between £30 and £40 for the pair. With £10 for the card, your 50 cards are likely to cost you 75p to £1 each. This is probably rather more than you spend on typical packs of charity cards, but the difference is the level of personalisation. You always pay more for bespoke products.