type LineItem = {Cost: decimal option; Price: decimal option; Qty: decimal option}

let discount = 0.25M

let createItem (c, p, q) =
    {Cost = c; Price = p; Qty = q}

let items =
    [
        (Some 1M  ,    None   , Some 1M)
        (Some 3M  ,    Some 2.0M   , None)
        (Some 5M  ,    Some 3.0M   , Some 5M)
        (None  ,    Some 1.0M   , Some 2M)
        (Some 11M  ,   Some 2.0M   , None)
    ]
    |> List.map createItem

items
|> Seq.map (fun line -> line.Price
                        |> Option.map (fun x -> discount * x))

val it : seq<decimal option> =
  seq [null; Some 0.500M; Some 0.750M; Some 0.250M; ...]

items
|> Seq.map (fun line -> line.Price
                        |> Option.map (fun x -> discount * x)
                        |> Option.map (fun x -> x - (line.Cost
                                                        |> Option.map (fun x -> x)))
                        |> Option.map (fun x -> x * (line.Qty
                                                        |> Option.map (fun x -> x))))

error FS0001: Type constraint mismatch. The type
    'a option
is not compatible with type
    decimal
The type ''a option' is not compatible with the type 'decimal'