RG6 is not a costly cable. The length involved is also 30 ft only, so the total cost will not be much (around $15). Why not replace RG58 by RG6 and see the result?
If anyone spends $15 on a 30ft section of RG-6 - they're getting royally screwed. You can pickup a 500ft roll of quality RG-6/U (quad-shield) on Amazon for $50. You can then cut the cable to the exact length you need. You'll have to supply your own fittings (of course), but even that is cheap.
Your cheapest option would be to find a Comcast/Charter/whoever cable tech somewhere around your town and ask them to cut you a 30ft length and install the compression fittings for you (crimp and screw-on coax fittings are garbage). I did this all the time for random people when I was a field tech for Comcast for almost a decade (easy to do when you carry around a few thousand feet of have hundreds of fittings in the truck). I also handed out splitters to people all the time as 99% of what you can buy at a big box or other electronics store is almost always junk - including the ridiculously priced Monster and Belkin goodies. Their cables aren't bad, but you're paying for the name and spending $20-$30 on the exact same cable you can get online for $5 without their name on it.
I really hate seeing people get screwed buying pre-made cables. The markup on cables is absolutely insane and retail stores and online outlets take full advantage of this. Monoprice.com is one of the few places where you can buy cables online and not be taken to the cleaners. Amazon can be cheap if you look around long enough.