If you go for an open source Portal solution like Liferay then the cost will be considerably less. Still as you said, you have to hire good developers to do the job.
And that might be expensive. Or you can use services of already trained/experienced consultants to do the job for you, if you are not hiring. But its always good to hire 2 good developers to do the job than 10 of them who are good for nothing.
But going for a Portal Solution offered by say IBM will mean quite a big investment.
You can do some case study which might help you in getting better idea. Like you can visit
http://www.liferay.com/products/liferay-portal/stories to do some case study for clients using Liferay.
Let me know if you have more questions.