To kick-start a collection that will give you options to build with, some very good products to start with are Bundles ($40) and Booster Boxes ($90). A Bundle includes enough land to make 3 decks, a box that can store or carry cards, and a booklet with images of each card in the set. A booster box contains over 500 cards - so if you select 36 of the good ones and make a deck (by adding 24 lands), you’ll be off to a decent start.
Another good option is to buy specific cards for a deck. Some decks are under $20 (search for “Budget Deck Tech”), but many of the really strong decks are more than $300. If you want to try investing that amount, there are web sites where you can pick a deck and check out all at once.
Perhaps you prefer to keep your investment closer to $50 until you have tried things out for a while. There are plenty of really fun, powerful cards that are extremely cheap. You can pick a powered deck, substitute out some dual lands and planeswalkers, and still compete for some wins.
There are several products that include a deck or two for about $30. These give an idea of the synergies between cards that make deck design a canvas that intertwines form and function. Some third-party retailers also design decks aimed at beginners, which can be even more affordable.
Playsets, which include 4 copies of each common and uncommon from a given set, are about $25 on eBay (500-600 cards). Playsets that include rares and mythics cost much more, but I think the first variety is a great way to invest in cards that will be fun to brew with.
Drafting is a good way to begin or augment your collection. You’ll usually end up with cards that work together for a deck, plus a few packs if you win some matches. All the cards you get at a draft will be Standard-legal, so you can build a deck over time and trade things you draft for others. This could also be a way to build a deck for Commander or Modern, although you’ll have to be pretty good to win without purchasing a few single cards as well.