>Just wondering if someone could advise me on the best action to take.
Spend the money and put in a cable.
If the buildings are close enough you might be able to run a cable aerially (cheaper). You could run a chain from one roof to the next roof (or if they are quite close you might get away with a thick catenary wire). Then run an outdoor rated Ethernet cable along the chain/wire.
>So what are the options available to us without the costs of installing a network point in the room?
Close the small building permanently. Ask those users to work from home.
Wireless repeaters have been mentioned.
If the bandwidth required is low you could also consider using something like an MX67C, and use a 4G backhaul, and use AutoVPN back to the primary MX.
https://meraki.cisco.com/product/security-sd-wan/small-branch/mx67c/
If you want more bandwidth, how about Starlink (flat-rate monthly cost) with an MX and AutoVPN back to your main MX or maybe a Z3 if the number of users is really low?
https://starlink.com/
https://meraki.cisco.com/product/security-sd-wan/small-branch/mx68/
https://meraki.cisco.com/product/security-sd-wan/teleworker/z3/