Money - depends on how fancy you want to build it. Structural parts out of 2x4's, screws, glue, etc. would set you back $10-20. How you skin it (ie plywood, solid wood, others), hardware, finish, extras like doors, shelves will drastically change the final price. For my 70 stand, I used solid hardwood with clear finish, and it set me back $200 or so. You can cut the cost significantly using plywood & paint instead.
Time - depends on your wood working experience & tool collection, and how pretty you want to make it. A basic stand can be put together over a weekend, but if you want to do furniture or heirloom grade work, expect to spend a few weeks.
Tools - minimum needed - saw, chisels, hammer/mallet, drill & screw driver, sandpaper, level & square, & tape measure. You can speed things up a lot (and usually get better results) using power tools instead. In that case you would need - table saw (good circular saw may be Ok), Power drill driver, Router, power sander. A planer/jointer & fancier tools will be nice if you plan to do fancy woodwork. Clamps are nice too for holding parts togather while you glue & screw, or you can get a helper.
Finally, you also need a flat & level space to build the stand. IME, trying to build furniture on uneven surfaces leads to uneven or out of level results. For a fish tank, you need the stand to be absolutely level & square .... although it is not as critical for a 20 gal
due to lower weight.