Weddingbee boards

It ranges, depending on what you want and your photographer.  If you only want a few general pictures for an announcement, budget little for it or have a friend take some at your setting of choice.  If you want to have a series of images to use/show for/at the wedding, and have a keepsake series of images to compliment your wedding images, then have your photographer do them.

If you choose to do an engagement session with your photographer, have in mind what you'd like, no different from what you'd want out of your wedding covereage.  (e.g. digital files, prints, signature mat, etc) 

Generally, it's great to do one.  Not just for the photographer and you to get to know each other either.  It's a great opp for you to know how your photographer operates and how he/she treats his/her clients.  Budget for any fees if you want to be at a specific location/park/venue that might charge them.

Below is a rough breakout of typical pricing you'll encounter currently, by no means comprehensive, but hopefully helpful:

note: any (a)...(e) sub points would be "or/and" depending on photographer.

1) Free; and clear as to what is delivered to you. 
   a) price is probably already built into your "package" 
   b) session might be shorter than usual sessions
   c) your photographer wants to build up more experience
   d) you'll know what it'll include and what you'll have to pay more for
  
   In all cases here, as long as you know what you'll get, it's great to have them done.

2) Free, or very cheap compared to your total photography cost.  *And not clear as to what is delivered to you.*
   a) just unclear communication, so same as (1)
   b) you'll not receive anything, or very little, and it'll turn out to be like a "free portrait sitting" but you'll have to purchase anything you'd like.  And the experience ranges from pleasant to car-sales-pressure-silliness.
  
   In (2b), I don't speak for other photographers, but imho, do feel that if you're not told upfront, such practices are bad and give brides/clients a bad taste of the photography industry.  And it'll also indicate potential sales pressure with your wedding photos/album down the road.  I think there was a thread "Engagement pictures: is this normal?"  where someone experienced this.  Avoid being here. 
  
  
3) up to $250. 
   a) shorter/single location session
   b) longer session depending on photographer. 
  
   This should provide you what you need in terms of files, a few prints.

4) up to $500. 
   a) more comprehensive session. 
   b) is (3) bundled with some product you might additionally want
  
   This should provides you what you need in terms of files, etc.  You should be able to have what you want, between (3) and (4), at about your photographer's regular portrait rates for repeat/frequent portrait clients.

5) up to $1000, and above. 
   a) specific to photographer
   b) specific to locations
  
   This generally includes additional items like framed/matted prints, signature mats, albums or guest signature books, etc.

Once you schedule it, have lots of fun, enjoy yourself!

Hope this helps and if you actually read all that....  here's a recent urban engagement session that'll hopefully inspire you to have an engagement session done:
Recent engagement session

cheers,
--siang
Lightphoria