The new beta of Auctioneer is not yet compatible with ReagentCost.

In order to make it compatible, first upgrade Auctioneer to version 3.1.0-beta4 from: http://auctioneeraddon.com/auctioneer/auct...3.1.0-beta4.zip

Once you are done there, load up ReagentCost.lua in a text editor (if you are not comfortable editing files, you can download a pre-editted version from my website:
http://norganna.org/reagentcostlua.zip

When you have the ReagentCost.lua file open, simply replace the FRC_AuctioneerItemPrice function starting at line 597 with this:

function FRC_AuctioneerItemPrice(itemLink)
    local itemID, randomProp, enchant, uniqID, name = Auctioneer_BreakLink(itemLink);
    local itemKey = itemID..":"..randomProp..":"..enchant;    
    local medianPrice, medianCount = Auctioneer_GetUsableMedian(itemKey, name);
    if (medianPrice == nil) then
 medianPrice, medianCount = Auctioneer_GetItemHistoricalMedianBuyout(itemKey, name);
    end
    if (medianCount == nil) then medianCount = 0 end
    
    local buyFromVendorPrice = 0;
    local sellToVendorPrice = 0;
    if (FRC_VendorPrices[itemID]) then
 buyFromVendorPrice = FRC_VendorPrices[itemID].b;
 sellToVendorPrice = FRC_VendorPrices[itemID].s;
    end
    if (sellToVendorPrice == 0) then
 if Auctioneer_GetVendorSellPrice then
     sellToVendorPrice = Auctioneer_GetVendorSellPrice(itemID) or 0;
 elseif Auctioneer_BasePrices
   and Auctioneer_BasePrices[itemID]
   and Auctioneer_BasePrices[itemID].s then
     sellToVendorPrice = Auctioneer_BasePrices[itemID].s or 0;
 end
    end
 
    if (buyFromVendorPrice > 0) then
 return buyFromVendorPrice, -1; -- FRC_VendorPrices lists only the primarily-vendor-bought tradeskill items
    elseif (medianCount == 0 or medianPrice == nil) then
 return sellToVendorPrice * 3, 0; -- generally a good guess for auction price if we don't have real auction data
    else
 return medianPrice, math.floor((math.min(medianCount, MIN_SCANS) / MIN_SCANS) * 1000) / 10;
    end
end


Hopefully this will be integrated into the next ReagentCost release.