It isn't all about the blade (although they are important) it's about how the blade is held and relative to what.
In a cheap plane you have a pressed cap iron screwed to an equally thin plane iron. The act of screwing the two together bends the cutting iron so that it beds on the frog at two points. The frog itself has relatively little surface area for the two points to bed on thanks to the big holes for the screws that hold the frog down on three little pads (often roughly machined) upon which it teeters, as there's three of them there must be three points of contact so it will find a stable position somewhere. Screw it all down good and tight and 'she'll be alright mate'. The sole that the cutting iron protrudes through will be flat to within a couple of mm or so, but if you're only using it to hog off waste material to within a few mm on warped pine whilst gorping at the passing totty and saying 'I'm a chippie me, innit?' and 'nah, beltsander job mate - that ones got a knot innit, innit?' a lot then you will be absolutely fine.
Alternatively if you ever want to be able to produce accurate components for furniture then you will need something a little more refined. Take this Clifton for example:
The cap iron sits flat on top of the cutting iron which effectively laminates it from above, the loose toepiece is accurately mated with the back of the blade but is an intentionally loose fit with the upper part to prevent any flexing forces from being transmitted through the system of components. This keeps the iron absolutely flat so that it makes full contact with the frog. The iron itself is left soft from the slot up and the cap iron is made from soft mild steel, all of which help to absorb and dissipate vibration. The double iron assembly is clamped down onto a bedrock pattern frog which has the adjustment gubbins hanging out of the back, not only easier to access but also allows there to be more meat where you want it - under the blade. The frog has a single flat accurately machined surface on the bottom, which sits on another single flat, accurately machined surface in the chassis. All of the iron components have been annealed over a two day period and then carefully machined and surface ground at low speeds to keep them cool and prevent stresses from building up in the casting. The sole has been precision ground to +/-0.00075" of flat (twice as good as British standard) in order that it will still be within British standard whether it ends up being used in Arizona or Alaska.
Personally I believe Clifton get the biggest percentage of important things right and they are far from being the most expensive. Lee Valley are certainly the most innovative, Lie Nielsen are the prettiest and Quangsheng are by far the best value for money (close to LN on design but pushing the boundaries with Clico on materials and processes).
The new Stanley sweethearts might be worth a look if you're skint - I get the impression they are starting pull their finger out after the public reaction to the first ones. The new Kunz ones look like a reasonable effort too but they should have started with a whole new brand rather than associating it with their earlier green efforts.
The cars analogy works to an extent ( I sometimes wonder whether Ferarri dealers get calls from people asking if their engines are available separately and if they will fit a 1984 metro without the need to file anything?) but to be honest, cars are much much more closely regulated than handplanes for safety reasons. As far as I'm concerned if it doesn't meet British Standard, it isn't a handplane.
