Most cost and calorie efficient fuels?

So I find myself in a weird spot, and am turning to the combined wisdom of the interwebs for advice: after a hit and run while cycling some years back flipped my life upside down I've spent most of the past decade food insecure. It's amazing what the body is capable of... at this point I can run marathons on one small meal in the three days prior, and routinely run marathons dry - no food or drink before or during the entire race. Because eating is no longer a 'normal' part of my day my gut needs serious retraining. On the few occasions where I've tried to fuel during races it has protested and tanked my run. In a half marathon last month I had to slow significantly because my gut gave me issues over a meal the day *before* the race!

For runs that take only takes 3 or 4 hours once I'm done I rehydrate, and sometimes refuel. For 50Ks, I'm fine with a little liquid. It's a very different story as I delve further into serious ultras. By mid miles I feel the need to take some calories, but the gut issues then slow the rest of the race. By late miles, I'm truly on empty and grit through on willpower. With full knowledge that its going to take significant gut training no matter what I do, I might as well do it right.

What economical fuel sources should I start doing some training with so that I can crush a few hundos later this year? Expensive commercial products aren't an option (unless its your company and you want to sponsor me.). Easy to carry, inexpensive yet efficient foods that are easy on the gut. It's a lot to ask for, I know!

Appreciate the advice!