I know it's not what you asked for but, I'm going to mention it anyway; a sheet of mid weight plastic.
Costs nothing weighs little and is quite flexible. You can make a tarp out of it by doubling the edges and trapping a stone in the doubled section and tying onto that by way of a guy attachment etc. I've spent quite a few nights whilst up the hill fishing simply rolling myself up in it in the gear I stood and lying on thick heather for insulation, never felt particularly damp or cold dossing like that, I think being open at both ends helps with ventilation. Perfect for a sheltered wood, the plastic, with a little care, will last a fair few trips.
For low cost cordage, for guys, I've just gotten a new roll of polypropylene 3mm mending twine, 300m for £8.50. Thats a lot of guys, not as fancy as "para cord" and not as nice to tie knots with but a fraction of the cost and it'll do the same job.
I've had several bivi bags in my time and they are great, but if I'm honest it's only when my life depended on it that I really needed one and that wasn't often. One time I was benighted in upper glen nevis in cold torrential rain. I found a boulder offering some shelter just as it was getting dark I then managed to get my then new bivibag out and my fiber-pile sleeping bag into it without getting it too soaked, then my soaked self. Woke in the morning both dry and alive (that was an early gortex bag) I think my trusty plastic sheet wouldn't have worked in those circumstances.
I've a stainless mora knife that I've not long looked out to sharpen, used to get issued these at work it's the model with the finger guard that extends out of the blue handle, I looked it up online and the price was still around £7 but I'd think the thicker 3.5mm carbon "heavy duty" blade would be a better option, but you'd need to stump up and extra £6 as these are £12.99.
So in my opinion there's no need for fancy kit. I would however spend good cash on a quality foam doss mat though, insulating from the ground should be the number one priority.