I have taken both Vertical and Lateral, starting April 2011. I passed Vertical in October 2011 and passed Lateral April 2012, basically taking them both in April and October last year and only Lateral in April 2012. It was a very expensive as I paid $2500+ for the exams and had to travel from CA to WA, etc. If you include the material I needed to purchase (as this year 2009 IBC was specified, AASHTO manuals x 2 , 2007 2010), and Kaplan SE preparation course, probably I spent between $8k to $10k.
However, there is a lot of overlap between Vertical and Lateral and if you can afford the added expense, $500+, it will be beneficial to take them both as even if you failed them, the whole process of taking it gives you a big edge next time you attempt it. This April, when I took the Lateral only, I struggled with issues, such as masonry or wood that were covered very well in the Vertical portion but I forgot about them since I was focusing only on Lateral.