I'm interested in building a computer (cheapest parts) that will be the cheapest for mining (not it's solo purpose). I would also be using this as a regular computer, but would like it to be expandible in the near future (expand number of gpu cards). Any suggestions on what graphics cards will go with a good motherboard for the cheapest price? 
If you want the cheapest you can buy the gtx1050ti. It costs around like $150. I highly suggest better not mine if you just have 1 gpu to do it. You'll just waste time and money and its not profitable. If you want to see any profits go for 1 gtx1080ti but this costs around $800 and still it will take you like 6 months ROI. Its really up to you though if your just experimenting on mining or just wanna experience mining using your computer at home.
Here's a link that will help you on your venture in gpu mining, hope this can help you out.
https://www.lifewire.com/cryptocoin-mining-for-beginners-2483064