I have several thoughts:

Weighing CPU validation cost should be reasonably straightforward-- just
pick some arbitrary, commonly-available, recent hardware and then benchmark
the two things that take the bulk of validation time (hashing to create the
signature hash, then ECDSA validation), and weigh the terms in the
validation cost equation appropriately (e.g. hashing X GB of data takes the
same amount of CPU time as one libsecp256k1 validation, so count cpu cost
of an OP_CHECKSIG as 1 + X/actual_bytes_hashed).

But how should bandwidth cost be counted? There isn't an obvious "Y GB of
bandwidth-per-month equals 1 ECDSA validation. We need to find common units
for the terms in the validation cost equation for it to make sense,
otherwise we're adding apples and oranges.

I think the only units that will work is "percentage of maximum validation
ability for some reference hardware running with a network connection
capable of some reference bandwidth."

For example, imagine the reference was the typical home computer being sold
today running with some multiple or fraction of the average global
broadband connection speed of 5Mbps. CPU cost to validate a block can then
be expressed as a percentage of maximum capacity, as can bandwidth--
hooray, two metrics with the same units, so they can be added up.  If the
result is less than 100%, then the block is valid-- it can be received and
validated in a reasonable amount of time.


Rolling in UTXO growth is harder, for two reasons:
1) UTXO changes per block can be negative or positive, as opposed to
bandwidth/CPU costs.
2) It is not clear how to choose or benchmark "reference UTXO growth"

(1) could be finessed to just treat UTXO shrinkage as zero.
(2) could just be decided by picking a reasonable growth number. Since we
want the UTXO set to fit into main memory, something a bit below the
long-ish term price/performance trend of main memory would be a good target.

So, starting with that growth rate and an initial UTXO size in bytes,
divide by the number of blocks in a year to get a maximum UTXO growth in
bytes per block.

When validating a block, take the actual UTXO growth, express it as a
percentage of the maximum allowed (make it zero if it is negative), and
combine with the CPU and bandwidth percentages.

If the total is less than 100%, block is valid. Otherwise, invalid.

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Now.... all of that worked through, I'm not 100% sure it solves the "do
miners or wallet have to solve a bin-packing problem to determine which
transactions to put into their blocks or what fees to attach."

I think it mostly works out-- instead of fee-per-kilobyte, it would be
fee-per-validation-cost (which is in the weird units "fraction of 100%
validation cost").

But the UTXO term might be a problem-- transactions that create more UTXOs
than they spend might end up being costly. I'm traveling right now, perhaps
somebody could pick some arbitrary reference points and try to get a rough
idea of what different transactions might pay in fees (e.g. if a
one-input-two-output had a cost of X, two-output-one-input would have a
cost of X/something).

I'm not convinced that a single validation cost metric is the best
approach-- it might be better to break the cost into three (UTXO growth,
CPU, and bandwidth) and just let miners set reasonable transaction
selection policies that keep each of the three under whatever caps are
imposed on each. If a miner comes up with a clever algorithm that lets them
pack in more transactions and get more fees, good for them!

But I do like the simplicity of a single validation cost metric.

-- 
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Gavin Andresen