The best deal I ever got was when a local hobby shop was going out of business way back in the 1970's. All of the carded 1/72 Airfix and Frog kits were being closed out at 3 for a dollar - 33 1/3 cents each. I bought several dollars worth

Pete

Back in the 50's, a model aircraft kit cost only 79 cents. Model cars in the late 50's, early 60's cost 98 cents. I got a 1/48 Scale Hasagawa F-4J Blue Angel kit, with white metal parts, at a yard sale ten years ago for one dollar.

Kind of depends on what they were. From the early 40's to the late 50's Comet produced a series of solid wood kits based on the goverment plans for 1/72 ID models used in WW II. You got a set of plans (three views and templates), a large block of balsa, rough sawed to side view, and a piece of balsa 1/8" x 3" x 6" with the outline of the wings, vertical stab/rudder, horizontal stab/elevators and prop blades printed in blue on it. They cost a dime at the local dime store and even as klutzy as I am at carving, I bought a bunch of them.
From the mid 50's to early 60's Comet and Lindbergh had a bunch of small kits (Scale? - we don't need no pocking scale!) which ran betwee 19 cents to 29 cents (had a whole bunch of them) Doesn't sound like much but to a kid at that time it was (A 3 bedroom house might cost $5000, a Vette cost $3000 and someone with a $4000 a year job was thought to be well off)
The best deal I was ever made was totally free (to me at least). About 25 years ago my best friend at the time, got divoriced and decided that his modeling had caused it. (it wasn't but you couldn't convince him of that) The upshot was that I ended up with all his modeling supplies, references and 350 models. Argued with him but was informed that it was going either to me or the city dump. Talked with him the other day and he hasn't built a model since. Oh well.

i pick up 1/144 planes for less 1.50 regularly.

I think my cheapest would be a 1/25 Chrysler 300c by AMT Ertl for about $3.50, and an 1/72 SBD Dauntless by Testors for $3.50.

wasn't me but my brother a bandai m30 cargo kit with just a few pieces missing at a garage sale for only $0.50

Too many kits to remember which one was the cheapest.

The rarest i think was the Pink Panther Car with interior & Carpet.

As for rare Kits we got one shop here that specialises in OLD Kits, plenty or Aurora Moonbuses and similar still on the shelfs pity is you need a fully charged Platinum card to shop there.

Berny13 brings back some memories... who was it had the famous "49'ers" line of models? That's right, they were 49 cents. Max price was .98. You would get a very cool brochure of models (line drawings) with every kit.