For the price, you might look for an old pair of Alesis Monitor1's. I had a pair, and they were decent for me. 

I'm not a fan of the krk stuff myself, but there are enough people who swear by them to totally dismiss them. as being useless just because they are not my cup of tea.  I can certainly say that i have heard recordings from people using them that sounds as good as anything anyone on this board has produced.  not sure how the hell they did it, but can't argue with results.

i'm going to second bitflipper's comment - get a nice pair of headphones. no matter what you do, for $150 you'll have to do some serious compensation using a lot of different speakers (laptop, car, earbuds) to make sure you are still in range.

would i mix on headphones alone? surely not. but they fix the most fundamental problem of all hobbyist recording: horrible room acoustics. you cannot mix what you cannot hear. headphones, if anything, will provide "too much" detail, and too wide of a stereo image, so that you think something sounds great but it really sounds like mush on speakers. however, that is something you can work on through ear training and using multiple sources as references. bad room sound is much harder to compensate for.

I use Equator D5's and a pair of ATH M50 headphones.  I installed acoustic foam in my room and as a result, my mixes are translating much better to the outside world.  Overall, this was a fairly small investment.  

If you can't quite afford something decent, go with a good pair of phones like Bit suggested.  Save your money and get some decent speakers when you can.  It will be totally worth it.

Rimshot

I'm with @Bitflipper. I'd get a decent pair of headphones or a used pair of hifi speakers.