If you can't afford it then I would recommend waiting. I know it is hard, especially when you read of people over in america who are getting scanned by 6 weeks, and over here you have to wait till 12 weeks. But if you have no pain, no bleeding, no problems, you just need to hang on! The first trimester is very anxious for everyone and I can tell you that having two early scans for medical reasons this time has not made me worry less really!
You will need the money further down the line for baby stuff so if it is too expensive for you to afford, I think you should give serious consideration to waiting. Thankfully it is rather rare for people to turn up to the 12 weeks scan and get bad news. Mostly if the pregancy has miscarried, it will happen before that point.
If you really want the scan, sit down with your OH and explain to him how anxious you are and that you want the scan to make you feel better. I think you need to make a joint decision on it though and not just ignore what he says and book it anyway, especially if money is tight for you.