Although I haven't played as much in the past year, most of my advice and links should still help you out.
Your first option is to buy a preconstructed deck. No matter where you buy it, it'll run $10-15. There are usually four or five precons released per set. They're typically built around the set's different themes and are a pretty good starting point. If you go with this option, be ready to fork out some cash for extra cards because while they're fully playable out of the box, most of them are greatly improved by adding cards. It may actually be better to buy from a local store for these as online retailers tend to mark up the price of the better precons (though it's usually not that bad unless there's a card that's used in tournaments in the deck).
If you're just playing casually with your friends, and can resist the temptation of having the latest and greatest cards then you should buy singles online. For most cards that have rotated out of Standard (those that are usable in the most common type of tournaments) prices per card range from $0.10 to $0.25. Rares many rares from older sets are $0.25 to $0.50 unless they're really good. Cards from current sets that don't see much tournament use are generally $0.50 to $1.00. Basic lands can be bought in bulk for a few bucks. All things considered, you can build some good to great decks for under $20.
The only problem with buying and building a deck solely from singles is that you already need a deck idea (Color, objective, theme?, specific mechanics?). I'd suggest starting by using the Gathering on the MtG website. It's a database of every magic card ever printed so you should probably limit your search to a specific color and set (or block) to avoid being overwhelmed.
I'm not sure how long it's been since you last played or how into it you were so I can't give any really specific advice so just ask if you have any questions.
Anyway, I usually shop for singles at
http://www.blackandbluemagic.com/ as their prices on older cards are pretty low and they have cheap shipping. In the past I've order from
http://www.shuffleandcut.com/ and
http://www.anycraze.com/ , but even when these two have lower prices, the margin is small enough that their higher shipping makes black and blue cheaper in the end.
For my last piece of advice, look into different versions of popular reprinted cards. Often certain cards are rereleased in each core set and you may be able to save a buck by buying a slightly older version.
I hope this helps and good luck!
