If your acoustic has metal strings, the absolute cheapest would be your acoustic, with a sound hole pickup. I have this one, and can confirm it works with Rocksmith. You would also need a female to female coupler, for it to connect to the Realtone cable.
But you'd be better off buying an electric.
This is the one that I picked up here from Amazon for only $13. It does the job. Then get you a female to female audio jack adapter and you're set. I found a 3 pack on Amazon as well.
> You can buy a Fishman TriplePlay Wireless Guitar Controller
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Or hit eBay and buy four beat up electrics for the same price.
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Guitar rig or a host of various VST plugins for DAWs out there are the better answer.
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You're not going to make your dreadnought into a vintage Les Paul in terms of laying out howling, sustain laden leads, but if you're wanting to lay down some filthy power chords, you can get the job done, more or less.
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https://producelikeapro.com/blog/free-daws-best-available-2018/
http://blog.reddogmusic.co.uk/2012/12/15/6-free-vst-guitar-plugins-thatll-blow-yourmind/
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When you do have some cash, go take a look at your local brick and mortar guitar shop. They're probably going to have a $100 guitar/amp package for you. Amazon has them right now, starting at about $85. After that, a cheap audio interface for your computer will make your computer much much better at handling the audio; the Behringer UM2 is perfectly fine for around $35, and you'll be amazed at the quality of suspended cardoid mic you can get for around the same $35 nowadays.
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Here's a bonus $8 solution: https://www.amazon.com/Neewer-Acoustic-Transducer-Microphone-Amplifier/dp/B00ADHKKZK?ref_=fsclp_pl_dp_1 Keep in mind that you get what you pay for, and you'll still need somewhere to plug in.
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Good luck!
For acoustic you can also get one of these $12 pickups that attach to the sound hole.
https://www.amazon.com/Acoustic-Electric-Transducer-Microphone-Amplifier/dp/B00ADHKKZK
get a cheap pickup like this one
and a instrument to USB cable like this one
Plus a $6 female to female adapter and your all set. Can play rocksmith with it or use amp and fx modelers on your computer using your acoustic.
I have this one, and can confirm it works with Rocksmith. I like electric guitars better, so I don't use my acoustic too often, but every once in a while I'll get it out. You need a coupler as well. I bought that one, but then it turned out my tuner had an input and an output, so I could have skipped it. Less than $15 for both of those, if you can swing the free shipping.