When the Wave 2 stuff (A-Wing, TIE Interceptor, Falcon, Slave I) was still kind of on the distant horizon, the stock answer I think you'd get from most anyone around here would have been:
2 x Core sets
1x Each of the expansions
Additional expansions to taste and preference.
Now though, I don't know.
I have a couple of friends looking to get into X-Wing within the next couple of months, and I find myself, after really looking at the rules for the new stuff, going with that stock above answer less and less.
I think once Wave 2 is released (in 4-8 weeks most likely), people should buy what they want to field. I mean, yeah, you have to buy a Core to get the rulers and whatnots you need to play the game (the rulebook by the way is available for download under the "Support" tab for X-Wing here on the FFG site), but as far as additional ships past the 1 X-Wing and 2 TIEs that come with the Core, I think it just really depends on what models you want to put on the table. All the ships in this game are viable and balanced for the points, and there are many, many valid builds that can be generated from the models. Some are easier to use than others to be sure, but having a less than optimzed list in this game isn't as significant as it is in some other miniatures games.
If you're going for straight up cheapest way to get to 100 points, dig this Rebel purchase list-
1 Core Set
The MSRP would be $40 (Core) + $30 (Falcon). You can kit up the Falcon and the X-Wing that comes in the Core set to be a 2-ship, 100 point list. Is it particularly competitive? I don't know, I kinda doubt it, but you could do it. For the same price (in money, that is) you could also do-
1 Core Set
1 X-Wing Expansion
1 X-Wing/ Y-Wing Expansion.
That'd be more flexible, though probably still pretty challenging for a new player. MSRP would be $40 + $15 +$15, or basically the same price as the Core and a Falcon. Just depends on what you want to field.
That said, it is really handy to have 2 Core sets. The extra templates are nice for teaching friends how to play, and the 3 attack/ 3 defend dice really kind of aren't enough to play an average game (for me, the sweet spot would have been 4 of each in the Core set). If you think you're going to get into X-Wing Miniatures much at all, I'd advocate for buying 2 Core sets still- I think the Expansion purchases are where folks need to stop heeding the old advice and consider Wave 2 preferences in their responses.
Good luck and have fun! This really is an awesome game!