I've messed around with a few options - a (real) cheap clamped-clip-on - didn't like it much - it worked - but was WAYYYYY too bassy with very poor high-end response and reacted to occasional "thumps" on the fiddle far too much - you know - when you're not actually playing, but holding the instrument and just moving it about in your hands.   This particular clip-on slides a tensioning arm into the F-hole and plonks the pickup directly onto the body.  

There is another clip-on type which is a simple audio style "stick microphone" - not much more substantial than a thick biro-pen.  I quite like that - it does need decent positioning, and it does have a tendency to pick up breath noise ( LOLOL and my cursing when things go wrong !!!!! ) - but - it is my preferred method for the acoustic right now.   I have NO idea how it might function in a "live amped up" situation - feedback might be an issue ( well so would the "clamped-on" one I guess, maybe even more so actually ).....

Both of the above are "relatively" cheap - the "clamped-clip-on" is only a few dollars (and I'd advise against it anyway).  The clip-on "mini-microphone" has various options - you may want it as a direct"line-out" for amping up, or via a mixer or FX box, or you might want a USB style mic for direct recording to an AWB or similar.

There are other types that actually slip a piezo transducer under (or into) the bridge - so - it's like a permanent fixture once done - but of course you can leave the end connector "floating" and unused for non-amped performances - but never tried that yet - one day....

Just some thoughts....

EDIT: I meant to say - check out the FiddlerShop site - there is quite a nice looking pickup that just clamps on to the bridge - the "Barcus Berry" - I had been looking at that one myself, but never actually went down the road - it's around 100 US dollars or so..... ( my "cheapo" clip-on was like 10$ and is now consigned to land-fill (LOL), and my stick-mic was about $50 )   The "bridge-contact" Barcus Berry would naturally remove all unwanted "vocal" pickup hahahahah (grunting, snorting, laughing etc... hahaha) that I get from my mini-stick mic....   But, because I do a lot of home recording and local mixing, and also "live" StreetJelly stuff, I also have a completely separate second set-up which is just a studio mic - USB direct to PC for my acoustic - so it picks up my playing and my chat to the viewres equally well, and I use a phantom-power ribbon mic via mixer/FX into PC for my EV (which is separately amped-up).  They are both EXCELLENT devices, at around £60 UK sterling.  Couldn't ask for better for what I need.  What you choose, all depends on what you want to do with it - my setup would not work for playing in a group, for instance...... (well, the clip on-stick-mic would)....