Cement is your friend
Go to your local Lowe's or Home Depot, and get several bags of premixed concrete. Example here, 80 lbs for less than $5.
Also, get a cement mixing tub, example here, for about $5.
Figure out how much you want each weight to weigh, and then multiply that by 1.2
For the sake of example, assume you want two buckets, each weighing 50 lbs for a total of 100. Multiply by 1.2, for a total of 120.
Whatever that number is, put half that weight of cement and half that weight of water in the mixer, and stir very very thoroughly until it's all in a liquefied slurry. You don't want to see any powder at all.
Continuing my example, that would mean you dump 60 lbs of cement in, and 60 lbs (or 7.5 gallons) of water in.
Remember how much you wanted each bucket to weigh? Multiply that by 1.1, and put that much wet cement in each.
In my example, I wanted 50lbs apiece, so I would put 55 lbs of wet cement in.
Some of the cement will slop on the ground or be hard to scrape out of the tub, so I always budget a little extra.
Let it set overnight.
Voila, you now have a much actual weights to work with.
Measure them both. This will never be exact, so figure out which one is lighter and put scrap metal or gravel in it to balance them. Now, you're good to go.
Note: it often takes a few weeks to fully finish curing. In my experience, weights tend to lose a pound or two in overnight curing, and level out at about 90% of the original wet weight.
Now suppose you have a weight bar, and you want to make weights that fit it...how do you do that with cement? No problem! All you need is some PVC pipe.
Measure the diameter of the bar where you want your weights to go. If it's 1" wide, go to Home Depot and get 1" PVC pipe. If it's 2" wide, your standard Olympic Bar size, get 2" PVC pipe. If you're using a 2x4, you should be able to do this with 4" PVC.
Before you do anything further, MAKE DAMN CERTAIN THE PIPE WILL FIT OVER YOUR BAR. I had to throw away my first batch of weights, because I got the wrong size and discovered it too late.
Now, repeat steps 1-3. But before packing the cement into your bucket, put the pipe in there first and pack the cement around it. Example here, from when I did it.
After the cement has finished setting up, cut off the excess PVC with a hacksaw, put the cement block out of the bucket, and knock out any cement that's dribbled in the bottom. Now, the pipe will let it slide perfectly onto your bar and you're good to go
If you want smaller weights, it's easy to use smaller buckets or plastic oil drain pans to make weights between 10-30 lbs. I used this and this to make 15 and 30 lb weights, roughly.
In general, 1 gallon of wet cement = ~18-20 lbs.
I do recommend buying a few 2-1/2 lb and/or 5lb weights just for consistency and leveling up, but they'll be pretty cheap.
Congratulations! For less than $50, you've got over 300 lbs of weight to work with