Front of the laptop calculation...

FAs are there being paid anyway so unless the trial shows the work load is so low 1 FA can be removed (and they can operate with 1 less FA) then we can keep the FA salary cost constant.

Take a KL B737-8 (max 186 pax) and fill it with 150 Y pax.

Pre BOB assume 95% partake of the free offer @ average €0.5 (some pax will be taking one or more bottles of wine). F&B costs €71.

BOB assume a pessimistic 5% partake of something, with average profit €5. That's an income of €37.5. Up that to 10% that's €75.

Difference then between pre-BOB & 10% BOB uptake is €146. Hardly big money and probably the ticket price of (assuming 4 seg RT of €200, €150 + gov. taxes + external fees (complete guess)) about 4 pax.

There would be other cost savings in that you'd only need to prep & load one BOB cart (if that).

If my guesstimate of the BOB uptake is somewhat low then there is room for a reasonable amount of extra income, which the trial would measure.