How to prepare a prenuptial agreement?

I need to get a really simple prenuptial agreement prepared. The objective is just to keep prior assets as separate property (and any income/proceeds/appreciation in value of prior assets) in the event of seperation and we don't care about any terms relating to the future. The situation is basically that I have a sum of money that my parents inherited from my grandparents and they put it in my care to invest. We now live in different countries and it would be very complicated to transfer the assets without a tax disadvantage. I just want to put their mind at east that it will remain available to them if they want the money. Unfortunately the lawyers I contacted in Toronto quote at the very least $500 + HST for something so simple. Alternatively, there are some legal forms that you can purchase for $25 (legaldepot.com) and adapt yourself, but I'm not sure if these are valid documents that would hold up in court. Any advice how to proceed?