Seems like a tall order, TBH, at least if you're talking about buying new retail.
Some possibilities, are an Asus AC1750 (I think same hardware as AC66U, which is supported by third-party firmware pretty well), REFURB from Newegg on ebay at $79.99. (That's the cheapest really decent / gaming-oriented AC router with gigabit ports I could find. I bought one for a friend, but we're just using the factory firmware, not a third-party firmware.)
Another possibility is an AC68R (same hardware as AC68U, definitely takes Tomato firmware, as I own a few of these personally), REFURB from Newegg on ebay for $119.99. (Bought mine when they were $79.99. One came with a defective AC adapter, which costs ~$22 on ebay for a new one.)
Some other alternatives, are a REFURB R6300v2 on ebay, from various vendors (not Newegg) for around $40. Supposedly similar hardware to the AC68R inside.
Edit: If you're wanted a strictly new router, then Walmart B&Ms (at least mine did) had some Belkin AC routers on clearance, but I think that they were more than $50.