Ive been in the market for this kind of stuff, and something I've heard is
1) 8x10 is the superior size/product for tarps
2) Borah gear is one of the best value makers, especially for the bivy and maybe for a smaller tarp.
Also 3) this is a decent amazon option that I found if you want it quick, but otherwise I would stick to cottage.
If you don't want to go the DIY route or the compromise route (Something like the Kelty Cosmic Down), you could get a synthetic quilte like the EE Prodigy
Grand total: $504/85.1 oz
Alternatively, go full Jardine!
Throw in the sea to summit bug net and polycro from above and a lot of free time, and you'll have just gained some serious trail cred showing up in home made gear!
Not bad!
You could save a few more bucks with a flat tarp like Sanctuary SilTarp from Amazon.
Get yourself a CRL - https://dutchwaregear.com/product/continuous-ridgeline/#type-and-color
And a rectangular tarp - https://www.amazon.com/Sanctuary-SilTarp-Ultralight-Waterproof-Backpacking/dp/B01E6454HO
Others can chime in but thats about as easy as it gets in my mind.
I just got this tarp on Amazon and so far it seems perfect for my needs. And it's got lots of guy points
I have the Paria Sanctuary and I love it! Its your basic 8x10, no-frills tarp. It's big enough for me and my SO w/ some gear and it has the the added benefit of being super cheap ($79) but also good quality. Their website says that they should have some more back in stock at the end of the month, here's the page
Also: I agree that you're likely to find 13x13 too big unless you're tarping with 3+ people.
Are you talking about the Sanctuary kit another poster mentioned?