After having a severe foot injury, take my word for it and skip the work boot thing and get proper MX footwear. I'd rather snap my femur in half than go through that injury again. Stick with footwear designed for the application. I have Alpinestar Tech 6 boots myself.
With that said, I'd spend more on gear if I were you. In addition to boots, goggles, gloves, and the aforementioned MX boots, I'd also look into a decent set of kneepads. I've limped around with bruises on both knees from a very mild get-off because I landed on my knees in rocks.
Now, you'll find that kneepads don't fit properly inside "normal" clothes, while MX pants are cut to fit them. MX pants can be expensive, but at this time of year you can find places closing out last years models for cheap.
I had elbow pads and a chest protector, but I just replaced those with a SixSixOne Pressure Suit, which incorporates chest protection, elbow and forearm pads, a kidney belt, and also a back protector. I wanted it because my elbow pads slide down when riding and this eliminates that problem. The back protector is also a nice addition to the mix.
Now, I admit that all of the above will total up to a decent amount of money. Personally, I'd rather spend a the money and laugh when I fall off the bike, rather than get up in pain. Not to mention that I find that I'm a lot more confident with the protective gear than without.
Check Motosports Outlet for cheap gear. I know they're running big clearance sales at the moment. You can also check E-bay for cheap gear.
To run down the prices:
Helmet: $100-200 (the cheaper ones are polycarbonate shells and DOT; the more expensive are fiberglass or kevlar or whatever and Snell and DOT certified)
Boots: $100-200
Kneepads: $30-75
Pressure Suit: $160
Pants/Jersey/Glove combo $100 (on sale, normally about $150 for pants alone)
Think of it as half the price of a set of Ping clubs. :)