$100ish, mine cost $130 shipped from Amazon (Singer Heavy Duty 4423 Sewing Machine [affiliate link])
What to look for:
I tend to look for good reviews and then look for features that suit me well. In my case, I wanted a basic machine that would be durable and able to handle thicker fabrics. The Singer Heavy Duty mentioned above met those needs for a great price point. I was particularly drawn to the fact that it had a metal frame. My previous machine was a cheapie plastic one from Walmart and it was horrible. For about $30 more, I felt like I got more than my money's worth with the quality of the machine. Another benefit? Drop in bobbin and simple button-hole process.
Sewing Machine Basics:
Nowadays, most sewing machines come labeled with numbers and illustrations on how to thread. This is probably one of the first steps you'll take.
After threading, you'll want to set your stitch length (top knob) and stitch type (bottom knob). For most clothing sewing, you'll use a stitch length of 2.5. The smaller the number, the shorter your stitch. Small stitches have the benefit of lending strength to your sewing, but it takes longer to sew, it uses up more thread, and they are a pain to rip out. Longer stitches (like a setting of 4) are great for trial sewing like try-ons or for use in gathering, but the stitching is too loose / weak to keep things together long-term.
For the stitch type, the straight stitch will be your go-to. Other stitches commonly used are zig zag for knits, the shark-tooth one (yeah, I use super technical terms) for blind hems, and the button-hole one (looks like a rectangle to the left of the straight stitch) for...well...button holes. :)
Do you see the Reverse button at the bottom left? Press down on this to stitch in reverse. I use this to reinforce stitching at the beginning or end of a seam.
Right above that is a tension knob. I can't say I've had to use this often, but here's a wiki in case you need it.
To the right of the tension knob is the needle position knob. I primarily use this for zippers or adjusting my seam allowance. For narrow hems, I'll use the rightmost setting. I generally keep the needle position in the middle for the majority of my sewing.
I hope this post was helpful! Please let me know if you have any questions!