If you confine your bitch in your house, your dogs and likely even your bitch will do everything possible to mate including digging at doors and walls. It's an intensive hormonal drive to procreate --they go off their feed and moods swing.
If possible, could you see about homing your dogs with a friend or kennel as her season is just starting?
Have you checked with the different vets in your area to see what the actual cost would be? If it's just 'friends' telling you, they might be exagerating costs. It happens quite frequently. :( Also your vet could be more expensive than another.
If you're in the USA/Canada, check with the local SPCA, Humane Society, or vets to see if there is a low cost spay program available to you.
It's probabl to 'survive' one heat without pregnancy if you take care, but it's still chancy and not a pleasant experience to go through if you lack proper facilities or someone to help keep everyone separate.
good luck,
toodles