Starting point is to figure that you will spend at least as much on the stand, fish and decorations as you do on the tank and filter. This assumes that you go simple on the decoration, DIY the stand and get some of the more common fish.
That said, you need to decide if you are going to attempt real plants. If you do, the lighting can eat a big chunk of change and there won't be much left.
Probably a better option for most of us is to go on craigslist or ebay and see what equipment is for sale used nearby. I am guessing that you can get a decent 55 with a stand and basic lighting for less than $150 if you can exercise some patience. Near me, there are only one or two postings a month that are over 20 gallons and some people think that used is worth as much as new.
You will still need at least $75 just for common fish such as guupies, platties, tetras, etc. Look over the package of used equipment with the idea that some of the equipment won't really suit you but might work for a while until you can afford to replace it. The stand style might not be what you would build but might do a nice job for you until you are ready to build.
When you look at a used package don't forget the less visible items such as the heater, fish food, thermometer, water testing kit, etc. These things can add to a significant amount if you buy them locally piece by piece. You can often beat the price at places like BigAlsOnline if you plan ahead and order everything you need at once, otherwise the shipping will eat up any savings.
To summarize, this is a way of thinking about it that might make an affordable system be more than a 20 in your stated price range. The specifics of what you would like in terms of style, decoration and fish is a very personal choice and I wouldn't begin to tell someone what to like.
