Wait CLOSER to frost date...let the purpletop seeds harden on the stalk....then plant your seed....purpletop will be going dormant at that time even though is hasn't frosted. Yes, rye does really well when late fall planted....very cold hardy....just don't fall fertilize with N and it will get cold hardy sooner.
Most legume seeds are expensive period! But with vetch, we get such tremendous biomass in spring that cost becomes moot....and is sets hard seed so will volunteer over a couple years (if you let some seed set).
For vetch seed over the net....greencoverseed or weltersseedandhoney....with shipping should work out to $2-2.5 per lb. You might check with the Farmers COOP in Tahlequah and Westville.....usually well under $2/lb at the one in Fayetteville (if you are a COOP member, then price is real close to wholesale). GreenSeed in Springfield is the COOP wholesaler...quality seed and it will be coated.
Small grains are around 70-85 lb in our mixes. We use about 8lb/ac vetch in the mix when smothering is the goal. 4 lb/ac for general purpose fall/winter/spring grazing mixes. 2 lb for winter graze/spring hay mix (hay guys really don't like thick vetch).Terminate vetch no sooner than mid-bloom in spring if planting something else.....or let it go to full maturity for full smothering effect. It does set hard seed so expect volunteer plants the next couple years. Those rates are general for my area.....you may need to go a little heavier or lighter for the same goal in your area.