Well I have had the same wondering but it depends, if you're just going to do it as a hobby, how long are you going to mine per day?  From what I've heard the investment will come back in about half a year from mining 24 hours per day in a pool IF nothing dramatically changes, which probably won't happen.. So if you want to invest as a hobby, I wouldn't go for it.

It really depends on what your goals are.  For the price, you will have a hard time ever breaking even on a single erupter - which is fine if you know that going into it.  You might be able to break even through mining something for a small period of time and then selling it (either asic or gpu).

If you want to dip your toe in mining and have any reason to want a higher end graphics card (gaming, cad, whatever), AND you already have a pci-x slot open and psu that can handle it, buying a 7950 new or used is a nice option.  You'll spend more, mine just a bit more than an erupter, but hold your value more.  Also, you can explore ltc mining later if you want.  If you want to learn the mining tools and be a part of the network that's a great place to start.  My 0.02USD.

There are three vendors who 'appear' to be decent - bfl (shipping, very very very slowly), KNC (no proven hardware yet, but looks promising) & bitfury (proven hardware, October delivery date).  That said, run the calculations and understand what your getting into - the days of making something out of essentially nothing (cpu, gpu mining days) are long gone.  Many ASIC buyers are spending 10s of THOUSANDS of dollars and HOPE to break even in 3-6 months.  That sort of processing power (they are talking TH now, not MH).  However, if your just in it as a hobby, want to run a miner and contribute hashing power to the network, great!

mining.thegenesisblock.com is a much more comprehensive calculator

regardless, 300ish M at the CURRENT difficulty will likely earn 1 coin after... never...  the best option (and even this isn't a good idea) is to have MANY MANY usb sticks working away at the same time.  Unfortunately that will likely never break even unless BTC climbs really high.  If your just looking to mine for the sake of mining, then it's a different story.  I bought 4 of these, sold 2 on ebay and I'm using 2 to play with while waiting for my 'real' hardware.

use this calculator http://www.alloscomp.com/bitcoin/calculator
with nowdays difficulties 37392766.0

I mined with 500-600 megahashes for a month about a month ago and got .3 of a coin so with increased difficulty and what not, I would say a long time? I was using Slush's pool, I gave up. I'm presuming you have a Block Erupter?