I had a similar problem finding travel insurance cover for my Aunt for her 2 week trip to Canada last year, she is also in her 70s. I didn�t realise that travel insurance for seniors was so expensive. I found a site called
Over 60s Travel Insurance It listed many insurance providers that give travel insurance for the over 60s and there are a number listed that cover people in their 70s One that was listed provided cover up to 120 years of age, although I forget which one. The price comparison was also helpful I think I ended up getting 2 weeks cover for about �85 which I thought was very reasonable!