The X - 5 was conceived in 2003 and designed and prototyped over 2004. It was designed from the start as a conversion kit for the xx4. At that time, the xxx4 was a fairly new car and xx4's were sitting around by the hundreds gathering dust; the average going price for a used xx4 roller was about $60 to $75. That meant that, even if you didn't already have a xx4, you could purchase one used + the conversion kit and have an extremely competitive car for around $310 to $325, which is a very competitive price. Obviously over the years the number of xx4's out there available to convert has dropped, while demand has stayed fairly high, and so the "overall" price for an X - 5 has gone up.
Around 2005, when the Losi team began running their xx4's again and the company re-released the car, we here at X Factory began receiving a huge number of requests to release a conversion kit for the xxx4. The car was never designed to be created from a xxx4 donor, but the re-release spurred enough demand for one. To make this kit, we simply took a xx4-conversion kit and added in all the extra xx4 Losi parts needed to make an X - 5. The price difference between the two conversions is simply our extra cost to include those Losi parts - we actually net less overall selling a xxx4 conversion than a xx4.
As to whether its still competitive or not, I still run my car regularly and do very well with it. The drivetrain takes some massaging to get right with today's lipo and brushless power (again, it was designed back in 2004) but once you have the X - 5 built properly its just an awesome car to drive.