If you use the tried and trusted stand design "template" that the vast majority of all DIY'ers use, it is very easy to calculate the "lengths" of wood you need. Wood "thickness" is dependant on tank size, but for a 125g then 4x2's are fine, if not overkill.
Once you have determined the lengths and thickness part of the build it is pretty straight forward to just go to the wood yard and get the amount you need.
If you are on a budget then knowing exactly how much wood you need is a big help, especially in the current climate where wood prices seem to be all over the place.
Once your stand is done, any extra cost will come with wrapping it if you choose to wrap it, so plywood will need to be factored in too. Obviously tools, screws and glue too, but I suspect you probably already have these in your garage or shed.
The king of DIY's very early tutorials on stand building are very good, all still on you tube.