Since this seems to be a hot issue right now, thought I'd add my 2c.
Change Rabadon's buildpath to [New Mid-Tier Item + NLR = Rabadon's Deathcap] 3700g
Have the mid-tier item build from Blasting Wand and Amp Tome, and give 60 AP and a % AP amp unique passive of maybe 10%. This item would have a high combine cost (perhaps around 700) so that it only becomes an efficient purchase when you already have a large amount of AP (~300).
Dcap itself would then have a low combine cost, but would still require the substantial investment into NLR as well as necessitating that the player already possess a large amount of AP in order to make the purchase efficient.