Helping my dog put weight on ????

hiya there

My dog is a cross labrador/ border collie n she is very underweight. We have taken her to the vets n they suggested putting her on a better dog food to increase her weight. She was eating bakers which they told me wasnt good enough(my other border collie is ok on this) so they suggested afew brands of dry dog food which when my husband went to buy this found it was very expensive.

I dont want to be awful to my dog but cant afford 45 pound a sack of food when it only lasts afew weeks. My hubby then found wagg (Working dog) which has the same contents as the other expensive ones so she is now eating that but i think she doesnt look any different from 2 weeks ago.

Does anyone know anything else i can do? ive wormed her.

She has loads of energy n the vets said this is why she buns off all her fat as she is a cross breed of 2 working dogs.

I feel like im neglecting her as she looks thin n can see her ribs. She is a happy dog though n now signs of illness

thanx keira xx