What can I use for a low cost effective border around flower beds?

How about bricks?

My mother collected rocks over the yrs., and they are now my flowerbed border. As you drive around your area, keep a lookout for interestingly shaped rocks and pick them up from the side of the road. In no time you'll have a free border.

If you want a short border, those foldable wire fences are good and cheap. If you need higher than that, Menards has a taller and more sturdy and decorative version that I use. It looks like fine black wrought iron.

Use bottles,old broken plates,cut up pallets ...alll cost effective

I like the ideas of rocks, wire or plastic fencing, and bricks. Each of these should keep children and dogs out of the flowerbeds, especially if parents and pet owners are paying attention. If there are any houses or other building sites around your house or work, you can ask if you can have some of their scrap and broken bricks. You might even find a pile abandoned that you can get. Best wishes 😇

Lawn timbers are reasonable.

A few houses ago and 30 years younger, I used broken bricks, rocks, pieces of broken concrete and set a short boxer in cement I mixed in small batches as I went along. Got lots of compliments and I liked that I used what would have been landfill otherwise.

my daughter-in-daw in New Zealand bordered her flower bed with small floral plates that were free at a municipal thrift store one weekend when they had an abundance and needed the space.

At about $1 each pine fence places will border a sidewalk, flower bed, etc. I use them at the back of my flower beds to block grass and weeds from coming through the fence of neighboring yards.