I appreciate you don't want to spend much - who does! But unfortunately, you do tend to get what you pay for and while you can get some cheap supp, this often equates to money down the drain, as they are not well balanced & generally not what they say they are on the packaging.
It depends what he's getting in his feed as to what he may need, so at least a basic diet analysis is a good first move, to make sure you're not wasting your money supping quantities he doesn't need. I use a (cheap) service/program called feedxl.com for this, and it's actually saved me money, besides saving me headaches & confusion, besides enabling me to keep my horses healthier than they've ever been.
With regards to price, while analysing heaps of different supps has led me to believe you get what you pay for, you may find the more expensive ones *per package* are not so pricey in reality after all. Eg. After using Feedxl.com I found a supp that would be perfect for my boys in their situation, but nearly fell over backwards when the feed store guy said it was $170/20kg!
But upon further calculations, I shocked him, by asking him to order it for me after all
- turns out I only had to feed about half the 'recommended amount' which equated to a huge feed of... 1 single handful per horse! So it actually works out to only about $1.20 per day for my 3 horses!