Let's say you put a microphone on an acoustic cello and an effects processor between it and an amplifier. Can you use this setup instead of buying an expensive electric cello?

Why are electric cellos so expensive anyway? Fender sells an electric violin (I suppose it's a good quality one, Fender is not a bad company) for 800$ something. Harley Benton sells theirs for under 300$. The cheapest Yamaha silent cello is about 2000$.

Is it true that the cello's bridge is held down by the tension of the strings? In that case it should not be difficult to replace the standard bridge with a pickup bridge?

I clipped a microphone on the bridge of my cello, hooked it up to an amp, and added some effects, and I was good to go.

Of course, you could do A LOT MORE with an actual electric cello (sound-wise and effect-wise) but I was satisfied with my method.

I am at a loss as to why you would want to do this!

Well, lets say you need to project your sound, a microphone must be used. You really shouldn't use a regular boom mic, because the quality is god awful on that, but a bridge mic, or an actual electric cello going through an amp will only pick up the cello, rather than the things around it, like a regular mic would do.

What I am getting at is, why would you need a mic on a Cello?? is acoustic not good enough? You surly are not playing pop or rock on it. ugh

Yeah, I know what you are getting at. When I used my mic, I was playing with singers, guitar, and an accordion. They were louder than me and drowning my sound, so I needed to amplify it.

You have entered a dark and dangerous world, be very careful

Are you kidding? The accordeon is verily the tool of the devil!
(grin)

On a more pragmatic note you can buy direct transmission transducers that clip into the wings of your bridge without harming the instrument and easily detachable.
The sound coming out of your amplifier or PA system will bear a slight passing resemblance to that of your cello unamplified, but not sound "real" unless you are prepared to invest a huge amount of money in a decent mic`ing system & preamp, that is as good as you are going to get.

Barcus Berry, Shadow and Fishman are names that come to mind.

I was thinking the same. Say no more.