I don't think going past 4GHz will get you very much performance. It's probably not worth bothering. (4.5GHz would be an extra 12.5% clock speed, which probably wouldn't help much unless you were very hard-limited by your CPU speed.)
That said, a good OCing Z77 motherboard would probably be another, like, $150-$200. (They're rare these days.)
An SSD, if you don't have one, would probably be a better choice.
I'm pinching myself -- to make sure that I really did remember some two threads in which posters asked this question, and both over the past couple months.
Right now, I find one board still selling at the Egg, which had some good reviews, good phase-power-design and OC'ing prospects, and isn't "out of stock:"
http://www.newegg.com/Product/Produ...re=Asrock_Z77_Extreme4-_-13-157-293-_-Product
If you look harder, you MIGHT find an ASUS board, but these AsRock boards have been the only remaining 1155 SB, IB -K -ready that I've found at resellers lately.
Someone else found some new boards at Amazon. Amazon also fields used-perfect-condtion or used-like-new boards.
Last fall, I found a bundle from another enthusiast for an ASUS Sabertooth Z77, i5-3570K and 4x4GB Corsair XMS3 1600's -- all at less than half of retail with a year's TLC wear-and-tear. Price-wise, the Sabertooth would be offered for "upper-mid-tier." Other boards you might look for persistently include the ASUS P8P68 -V Pro/Gen3, the ASUS P8P77 -V Pro or Deluxe, or someone offering a never-abused Maximum IV Extreme or "MIVE" or Maximus V Extreme for half what they'd paid for it -- or less (just to come close to your own budget target of $100.)
Finally, I bought a P8Z68-V Pro/Gen3 with the final BIOS edition, in the box but missing the I/O backplate, for $85 at an outfit named AscendTech.
AscendTech develops a corporate clientele and customer-base so that it can acquire surplus IT assets, damaged units and full-functioning items alike. It then tests, refurbishes, and sells at a different storefront -- to you and me. Just the photo theme at their "green" web-page explains it all. The other store-front is a tab on that page.
I checked
www.ascendtech.com recently for Skt-1155 Z68/Gen3 and Z77 boards, and there aren't now any offered for ASUS, but it changes week-to-week. There were some ECS boards available. I didn't look for Gigabyte, AsRock, MSI etc.
Also, look for any m-ATX board versions for the aforementioned model lines. Last year, there were some good bundles from small computer shops on E-Bay. There's a risk to buying used items, but if you take your time and hunt patiently, you can reduce that risk and maybe get your board for ~ $100.