Going just on the basics, marriage in the US - his plane ticket, your spousal visa. Marriage in the UK -- your plane ticket, your fiancee' visa, your FLR/spousal visa. So ultimately, marrying in the US would be cheaper.
Based on what you've written, I think the best option would be to have a small civil ceremony in the US (his family can come if they want) and get your spousal visa. Once you're in the UK, you can have a blessing and a reception. If you do it this way, you'll enter the UK with the ability to work (which you wouldn't have as a fiancee'), so that would help with finances, if they're an issue.