Hi,
The initial question was "how to get better results in mixing" so unless you are living next to a sawmill, soundproofing is irrelevant. You want your setup to be as neutral as possible so your mixes are balanced and nice to hear outside of the room you're using.
Each room sounds different so it is not possible to give an accurate, relevant advice on how to spend your money for the best results without actually hearing your setup in this particular room. However, you mentioned that the room is small (how small?), which implies that you inevitably have low frequencies response issues, even though you might not have noticed before. It is always the case in small rooms, especially if you have lots of right-angled corners.
You might have other issues like flutter echo, too many unbalanced reflections etc. but if your room is small, it is very likely that the main issue that needs to be addressed more than everything else is low frequency response. Low frequencies have very long wavelengths (for instance, at 40 Hz which is roughly the low E on a standard electric bass, the wavelength is close to 9 meters) so a small room just can't reproduce this naturally. Things get worse if the room dimensions can be divided by the same number (or each other), like a 3m x 3m or 2.5m x 5m room for instance, because room modes get crazy.
It is also likely that your monitors are close to a wall if the room is small, which only make things worse (especially if you have ported monitors, which is probably the case if you're on a budget). If you want to get an idea of how bad the low frequency issue is, take listen at
this. It is a chromatic scale of sinewave tones spanning 24Hz-262Hz, useful for hearing the bass response of monitoring systems, as well as distortion and turbulence anomalies.
My advice would be:
1. Spend the greatest part of your money (if not all) on bass traps. You can never have too much bass traps in a small room, while you can easily achieve a unnatural sounding room with too much acoustic panels that deaden the sound and traps too much mid and high reflections.
2. Buy good headphones and use them often in addition to mainstream monitoring because this will bypass the room "color" issues.
3. In the beginning, listen to your mixes in a wide variety of places and sound systems to get used to your "sound". You'll probably find that you tend to boost too much a particular frequency range, and vice versa. Once you've identified this, you can self-correct and eventually you will get good mixes in your (presumably) bad sounding room.
Cheers.