If you are American and what State you live in a pre-nup agreement can be mostly useless. In most states the agreement has to have limits and VERY detailed. A pre-nup agreement will not keep you out of family court... your previous spouse can still challenge the agreement... and often times judges rule in favor of the woman.
If you want an iron clad agreement you are going to have to pay a GOOD lawyer that knows what they are doing at least 10K (likely double that), since at a minimum a week's worth of work to pull it together and that assumes you declare ALL your assets you want to protect and you go in knowing EXACTLY what you are doing. Any ambiguity will take more time and be more costly.
If your pre-nup says something like "What is mine is mine: yours is yours... and if the marriage ends we take our stuff and go separate ways." Well that isn't worth the paper it is written on. Problems with that is that you have not SPECIFICALLY identified what is yours... you can not have an open-ended agreement with no time limits. You have to define if any assets you have is growing in value you keep the growth. In most cases... say you have a mutual fund worth $300K, if you put any more money into that fund while you are married, or move that mutual fund to another account, it then becomes a joint asset.
I had a friend that had a pre-nup. When he married he had his own house, and all his investments were with a financial advisor. His spouse, after they were married, sold his home (which he owned outright), and bought a new one. Moved his assets to another FA firm (from Edward Jones to Northwestern Mutual). He had a pre-nup... but because he sold his house and bought a new one after he was married, the equity in his own house became community property. When he moved his investments to a new company, those became community property. After 18 months of marriage he ended up losing $500k to his former love goddess. On top of that he had to pay her legal bills, basically paying the lawyer that ripped his pre-nup to pieces. Not a bad payday for a former waitress for a year and a half of 'work'.