Electric fence is a GREAT idea, can get from ebay and resell once you have finished or if it doesnt work.
We have one around the veggies and our kelpie wont even go in that part of the yard - the fence has been out of batteries for weeks. Total cost new at bunnings was $120ish
I saw him going under the fence the first time and the elec wire touched his ear and YELPED!!!! and ran off and wouldnt come any where near after that. Its not too much electricity, we ahve 4yo touching it all the time for giggles