I'm with
tlindsey
on this; adding new fish without quarantine is Russian Roulette. Sure, you can get away with it a few times, or many times....but eventually you will pay the price. When...not if, but when...you casually toss in some new fish that bring with them some virulent pathogen...well, it doesn't really matter how often you managed to get away with it up to that point, does it?
A small quarantine tank doesn't need to be expensive or elaborate. You can keep it set up all the time, or you can just set it up and install a small sponge filter that you keep mature and ready in your display tank until needed.
I can see the possibility of space being a problem, but if you put your mind to it you will locate some small cubby somewhere that you can use for a QT. Having it remote from the main tank is actually a good thing, reducing the risk of cross-contamination.
Don't have the money? Then you also don't have the money to burn on fish that you buy and which then die and take a bunch of your established fish with them, simply because you didn't want to quarantine.
A clean, well-run LFS is like a hospital; a poorly-run one is more like a medieval leper colony. Both places have a constant stream of sick individuals coming in; one takes all possible precautions, the other takes few or none, but if you need to visit one or the other it makes sense to carefully wash and sanitize your hands afterwards, before you kiss your wife and play with your kids. Same with new fish and quarantine.