A DGX Spark bought to resell tokens at $1 per million, at 20% duty, does not pay back. A box you already own only owes the electricity.
Street price comes back only if the tokens, at $1 per million, beat the watts. A Spark bought to resell that rate does not.
The clock
The payback table on Payback uses the US street price, residential power at 18.34¢/kWh, $1 per million tokens, and 20% of the day serving. Months are the street price divided by what is left after the power bill. Watts follow the work.
Fast
The top row is Qwen3.5 2B on an RTX 5090: 351 tok/s, $4,639, 575 W, 2.3 years. That print is Qwen3.5 2B · RTX 5090 32GB. Next is an M4 Pro at $2,199 and 45 W. Qwen3.6-35B-A3B at 85.5 tok/s is 4.2 years, Qwen3.6-35B-A3B · M4 Pro. Qwen3.5-4B at 82.8 tok/s is 4.3 years, Qwen3.5-4B · M4 Pro. The mini is fast because the watts are almost nothing. The 5090 is not a cheap card. It is fast because 351 tok/s still clears 575 W at $1 per million.
Slow
One DGX Spark is $4,999 on the street table. Qwen3.8-Flash-Next at 79.5 tok/s and 240 W is 12 years, Qwen3.8-Flash-Next · DGX Spark. Two Sparks at 94.4 tok/s and 480 W are 23 years, Qwen3.6-35B-A3B · 2× DGX Spark. Two Sparks at 45 tok/s are 77 years, Qwen3.8-Flash-Next · 2× DGX Spark. An RTX 4090 at $4,495, 40.7 tok/s, 450 W is 40 years, Qwen3.8-27B · RTX 4090 24GB. The power hour eats a weak token hour.
Cheap, or already yours
The cheap US price on this desk is the RTX 5060 Ti 16GB at $614. Its best quoted decode is 23 tok/s, 180 W, 7.0 years, Qwen3.8-27B · RTX 5060 Ti 16GB. The 10 tok/s uncensored print on the same card is 117 years, Qwen3.8-27B TurboFCFusion · RTX 5060 Ti 16GB. A cheap card pays back only when decode is high relative to watts. A box you already own has nothing left to pay back but the electricity. Buying a Spark to resell $1 per million tokens at 20% duty is the long case. The full rank is Payback.