Casting is pretty simple, think concrete, just instead of Portland Cement you use Calcium Aluminum Cement, which is much better for high temperature. Normally your Calcium Aluminum Cement would come in the form of a commercial castable refractory. A commercial castable refractory will have the calcium aluminate cement, proper aggregates and special ingredients to allow moisture and steam to escape. The cheapest one I can get locally(perfectly good to use for an oven) has a yield of 124lbs per cubic foot. It costs $27 for a 55lb bag. To figure cost you would need to come up with an oven design, find how many cubic feet it is, and then do the math, but that is way ahead of where we are now. I can tell you a small low mass oven that would not be suitable for commercial use would be about 3 cubic feet. My math says that would take 7 bags at a cost of $189 for me locally. I personally would also add stainless needles. They act like rebar in concrete, and work well at 2% by weight. I'm in a big city and still couldn't source them locally, but they are easy to buy on ebay. The example oven I mentioned would need about 10lbs. With shipping that's $35. So now we are at $225 for a small low mass non commercial oven, as I mentioned before for a commercial size oven I would guess you will double that. I'm guessing that doesn't fit your budget from what you've said. You could make your own castable refractory mix, but you would be forging your own path. Locally I can get an 80lb. bag of Calcium Aluminate cement for $60. If you have a source for old brick, you could use crushed brick and sand as an aggregate. I'd bet you could cast a commercial oven for $150 if you have a source for free old brick. It would lack the special ingredients to vent moisture and steam which would probably lead to more cracking, but the cracks would be aesthetic, all cast refractory will crack regardless.
Take this all in and if any of it sounds like it could work for you we can help you figure out the next step.