Steve B wrote:

I bought one of those random repeat forms that end up with a paver
walkway of what appears to be random sized stones, but there is a
repeat in it. You get your base prepared, lay the form on there,
pour concrete in the holes, pat it down, and immediately remove the
form.

If you don't mix the concrete correctly (ie - if you use too much water)
then the minute you remove the form, the concrete will ooze together and
the gap between stones will disappear.

You have approx. 2 sf of pavers.

Assuming this form is 3" high, then 2 sq ft x 3" is about .5 a cubic
foot of concrete (without taking the gaps into account).

Anyone ever use one of these?

I haven't.

It is surely going to take a lot of 80# sacks to do this job.
Suggestions/caveats appreciated.

I've mixed an estimated 420 cubic feet (15.5 cubic yards) of concrete
using a small 1.5 cf electric mixer over the past few years. I buy sand
and stone right from the quary and load it into my pickup truck myself,
so it costs me about $20 a ton for 1/2" crushed stone, and washed brick
sand is twice that. Cement costs about $12 a bag (40 kg or 88 lbs).

If you buy sand and stone already pre-bagged as 30-lb bags, then it's
going to cost significantly more to make concrete.

I also use pigments, super-plasticiser and air-entrainment agent, but I
won't take that cost into account here.

Taking only the cost of sand, stone, and cement into account, I figure
it costs me about $3.42 per cubic foot to make cement (equates to $92 a
cubic yard).

So going by those numbers, it would cost roughly $3.50 to make enough
concrete to do 2 of your forms (4 square feet).

You can make about 5.5 cubic feet of concrete with one 80 or 88 lb bag
of cement using a standard mix formula, which would be enough to fill
your form about 7 times.

Your average paver might be 6" x 6" and it would take 16 of them to
cover 4 square feet, and they'd have to cost 22 cents each to be
cost-equivalent to the concrete-in-form method.

The form method has a nice advantage in that your top surface will be
nice and even and flat, something that can take a while to do when
you're putzing with pavers. But you have to get the consistency exactly
right if you want to remove the form right after you pour the concrete
and not have the gaps get filled in right away. Even then, I would
suggest you have the sand ready to pour into the gaps when you remove
the form.