Depends on solicitors costs and agreement with your ex in all honesty. Also whether you discuss it with him yourself or use a mediation service as that's an extra cost. Then on top of any of that the court submission fee is £45. That makes it sound cheap, but it is the solicitor and mediation costs that are what you really need to know.
Most solicitors offer a fixed fee round here, if it is straightforward and you can amicably agree things, for around £1k. Mediation services could be an extra couple of hundred pounds - you have a meeting on your own, as does he and then one together. Solicitors costs go up though if you end up with letters going back and forth between his and your solicitors arguing the points. Always ask the hourly rate costs in this instance. Bills can sky rocket.
I know that there are online websites offering them for £125 say, but I don't know of anyone that has used them so can't comment. Maybe others can?
The reason for having a consent order is to settle all financial claims against each other now in terms of who has what/pays what, but more importantly so that neither of you can come back to the other in the future for any reason. Without one that is always possible. I am not sure what the added complications there are if there are already children of the marriage.
Have a look on Wikivorce if you haven't already - whole site dedicated to such issues and you can read forums there specifically about this. There is also a free advice line. I am not associated with them; I just used it when naive about my own proceedings! It also should never replace proper legal advice from a solicitor.