just my two pennies worth...
I bought my steel in 7.5 metre legnths and i cut them as i need them.
I worked out that i could buy a nice rage mitre saw and bits and bobs such as hacksaw blades and grinder discs for the same price as a precut kit.
I decided to go down this root because i really didnt want to make an error in my measurements ( im more of a blacksmith than an engineer
) and end up with a piece of steel that was say 1.5mm short and have no steel left to remake a tube(although i have been known to bridge gaps with weld, not reconmended
).
With regards to cutting i have cut most of my tubes with a hacksaw and iv only used the rage once and that was for the evil 74 degree cuts on the bottom of the frame and the compound mitres on the front frame. I could have done these with the hacksaw anyway so i suppose iv wasted a £120 on the chopsaw.
The precut kits available from a couple of guys on here are well renowned for being spot on. I suppose if i had known how long it takes to cut the tubing and how knackered i am from hacksawing i might have bought one, but i feel i have accomplished a great deal from turning a few legnths of scrap into a sportscar chassis.
So in the end cost wise it doesnt matter wether you go for a a precut kit or cut your own. The chassis is the most important part of your build, yet the cost of the chassis is a fraction of the cost of a fully built roadster. Going with a precut kit will save you a lot of time, wheras cutting your own steel will take more time, but you could use the savings for other bits, but you may end up with carpul tunnel syndrome like me!!!
hope this helps
nathan