I imagine it will be very expensive relative to a cross-canister call. Instead of just routing a message across a subnet you will have 13 computers make an outgoing HTTP call and then process whatever result is returned, then run a program to sync the responses and ensure a cohesive answer.
According to https://internetcomputer.org/docs/current/developer-docs/deploy/computation-and-storage-costs/ a cross net call’s flat fee is 260,000. So an HTTP out call is 1,538x for just the flat fee. The per byte for HTTP out calls is 100M per byte, but the xcanister is 1,000 per byte. So your bytes are 100,000x more expensive. Perhaps @dieter.sommer can give us an update on what the final costs ended up being as this was rolled to production…these were numbers from a beta discussion.