Give what you can afford. At our wedding we had a cheque for £200 from someone who wasn't even able to attend the wedding - but he earns £100,000+ annually, so it was small change to him. My parents' middle class friends and senior members of the family gave about £50, but most of our twenty-something friends and siblings gave £20 - £40, and that includes siblings and first cousins. Younger people, especially those with families, do have different priorities to older people who have no children at home or have paid off their mortgage, and you shouldn't feel pressured to give more to 'keep up appearances'.
I would have hated to think that people had put themselves into debt for our sakes. If your partner wants to help with the wedding, couldn't she offer her time / services in some way? I was really grateful to a friend who helped by altering the bridemaid's dresses, and was really touched when my sister in law's partner offered to sew a tear in our ring-bearing cushion. (He's in the army and did a much neater job than I could!)
Likewise, when my sister got married we didn't give them a huge present - but I bought her a bridal handbag to carry her tissues, and likewise she found one in a charity shop which matched my dress exactly for my wedding.
Instead of a lump sum, perhaps her help and support could be more valuable?