Others are saying that bell peppers have no taste -- but I'd disagree.
The green ones have a kind of grassy quality to them.
Obviously, there's wheat grass, but I never have that around.
There might be some herbs that could pull it off (eg, flat leaf parsley; maybe taragon, but also gives a licorice note)
You might try some dark greens ... spinich, kale, swiss chard.
They've got a touch of other stronger flavors to 'em, but if you cook them down first, you might be able to tone them down some.
(I guess you might describe it as a 'spiciness' ...
it can be a sharp flavor. ... for the texture, though, my first thought was bok-choy.
It's actually a cabage, and it has some dark green leaves, but the thicker white parts have that watery/crunchy quality that other people have mentioned about bell peppers, but there's that sharp cabbage note that might keep it from fitting your requirements.
update :
okay, as the issue is specifically fried peppers, you're going to be bringing out the sweeter notes of the peppers, which you're not going to get from a cabbage.
Sweet onions might give some similar qualities (once fried well), as the cooking will mellow the oniony qualities, but the sugars will caramelize.
You can also try looking for jarred peppers -- they're usually packed in oil or a vinegar solution after being roasted.
There are 'hot' and 'sweet' varieties, where the hot ones also have some hot peppers in there.
The non-vinegared ones sweet ones might work for you.
Also, remember that bell peppers, like most vegetables are a seasonal item -- if you're in the north, it's the end of winter, so any peppers are being being grown in hot houses, or brought in from the southern hemisphere, which jacks the prices up.
cooking.stackexchange.com/users/67/Joe
I find that bell peppers are a predominately watery and crunchy.
As such, I'd look for other vegetables that have that characteristic.
Depending on how much you are cooking them, you could use carrots, red or white onions, or celery.
I think those are all pretty close.
To get a little further away from the texture and flavor, you could also try broccoli, asparagus, or green beans; although I'm not sure any of those will be cheaper than bell peppers.
Depending on your climate, peppers are actually quite easy to grow too (at least in my experience).
If price is a significant issue, you could try some of your own gardening.
cooking.stackexchange.com/users/1259/yossarian
Nothing will ever replace the taste of fried bell peppers.
And I mean nothing, if the price double then you pay double.
Simple as that.
cooking.stackexchange.com/users/4086/Zombies
Watery and crunchy could also describe water chestnuts, or to go a bit further afield, bamboo shoots or even baby corn.
These all tend to be inexpensive, since they are purchased canned.
cooking.stackexchange.com/users/1887/Martha F.
Can't eat peppers any more (red, green, yellow etc) in chili I substitute with green squash.
No one yet has noticed.
cooking.stackexchange.com/users/85519/Christine Luciano
I haven't found any thing that tastes like a bell pepper.
So, I collected the seeds from one I purchased at the grocery store (well over 50 seeds)
Bell pepper grow well in pots even on my tiny deck.
I picked and froze/dehydrated 15 peppers from just one plant!
I saved those seeds too.
It only cost me a slight increase in the water bill.
This year I planted three.
Dehydrating changes the texture but saves the flavor and space.
cooking.stackexchange.com/users/76161/Marye Rusack
I recently became addicted to the taste of fried bell peppers (green specifically because that's what's cheapest) as an ingredient in meat dishes. However, in my area the price has doubled so I can't buy as many.   What are some things (if any) that can replicate the flavor (and the lack of spiciness) of bell peppers? Specifically I like to add them in beef and make sandwiches with cheese and onions.
Substitute for bell peppers in meat dishes
cooking.stackexchange.com
<substitutions><bell-peppers>
cooking.stackexchange.com/questions/12562
cooking.stackexchange.com/users/94/Kryptic
Some herbs and dark greens could work, or you could try caramelised onions, carrots and celery, bamboo shoots, water chestnuts or even baby corn. Try herbs like parsley, fried onions or water chestnuts and baby corn.
You could try some herbs like flat leaf parsley or tarragon. You could also try fried onions, swiss chard, water chestnuts or baby corn.