Falcon Heavy is a lot less than 1/8th the cost of an SLS launch. The number you hear most for SLS launch cost is $4.1 billion, but that includes Orion. SLS without a payload costs $2.8 billion to launch. Falcon Heavy is listed as $150 million for a fully expendable launch and that is an old number so it could have gone up since then.

Comparing those number (even if we assume a higher price for FH) looks bad enough for SLS, but remember that SLS launch cost does not include development costs. Development costs are tens of billions and counting, paid separately. Falcon Heavy was developed with private money, the launch price includes both the actual cost to launch and a operating profit so that they can recoup development costs. If you include a portion of development costs in the price of an SLS launch it would be billions more.

We don't know how big a portion of development costs to include in the price of each launch, but it is safe to say that SLS costs at least 20 times as much as FH.