Before I go any further…have you got a reloading press? That will make an immediate difference these days. A Mec 600 jnr (the type I use) currently retails at about £365.
If you’re looking at reloading small quantities, if you can find one on an auction site (a huuuuuuge if) Lee produced a loader in .410 (though you’d need to reassess the powder scoop measure), though I don’t think the shotgun versions have been made in 30+ years. These are great fun to use, but very laborious.
For 14g I assume you mean 2.5” shells.
Work on the basis of the following approx costs
1000 primed 2 1/2” hulls £155
1000 H21 wads £40
10kg #7 shot £47 (so 1.4kg around £6.60)
Hodgdon 110 powder 454g £45 (so about 16 grains per load equates to about £103 to load 1000 shells).
So it’s roughly £305 in components alone to load a thou shells 14 gr shot. That’s 40 boxes. (£7.63 a box of 25)
The cases should last 3 or so loads if looked after, so costs will reduce for the 2 subsequent loadings to be replaced by £62 per thou of CX50 primers.
You can buy a thousand say Express 2.5” for about £430 (£10.75 a box based on a thousand rate), so loading is a bit cheaper - if you have the tools, time and patience…
Obviously, this is whilst we can still use lead.