While I won't argue that the price of the Monbento is in the realm of sane, they are absolutely fantastic. I own two of them and the quality of the boxes still makes feel like it's a top notch product. I've put all sorts of foods in them and treated it like I would any other lunchbox and it has held up to whatever abuse I tossed at it (something I couldn't say about my old Bentgo).

The problem I see with what you're hoping to do is that bentos are not typically air tight. There are not many foods I can think of that last well in a nearly open container for more than a day or two. So what I would suggest instead is investing in a set of really good storage containers (I have an old set of Lock&Lock containers and they're awesome) and keep your cooked stuff in there, then toss them into your bento the day of use. That will keep your foods fresher longer.

If your'e still set on that, those Bentgos really are the best price you're going to find for a stackable bento box that isn't complete garbage (in my experience). That isn't to say that there are no options which may be good (such as this box I for around $11, or this one for $17), but after being burned on the budget ones in the past, I can't say I would recommend any of them.