Econonmy speed - not all that much of a fantisy - up to a point.....
As Dave noted, the Q-500 route is a pretty quick way to speed.
One of the ARF quickies or a kit built one would be ideal. Any of them, set up 424 racing configuration will run at about 120mph (ie: Thunder Tiger 40 Pro, stock exhaust, 9x6 apc prop, 3.5lb aircraft - engine generally set up at around 16-16.5 rpm on the ground). You can utilize your full size sport servos, rx, battery equipment. For sport use, the plane will not mind an additional 4-6 oz. They fly fine at 3.75lbs. The planes are easy to build, very smooth and well behaved, aerobatic, and land slowly if trimmed properly.
Same aircraft with a TT46 pro and a 9x7 (at about 16K rpm) will get up closer to 125-130. Any of the inexpensive "50" size engines (mag, TS, ASP, etc) will perform in this range too.
The Q-500 designs with an LA40/46 (properly broken in and set up well) will approach 100 mph on a 9x6. Maybe a bit more on a good day. There were (are) some regional racing series that used the FP and LA engines as their spec engines.
A Diamond Dust with as little as a .32 size engine can get over 100 mph. I would imagine you could do better than that with an LA40. Some of the Dust owners here can give you some speed information with the sport engines.
Beyond that......we may cross the line of an 'economy' setup.... (lots of engine and aircraft combinations)
Getting up to 150 mph requires some power and the right aircraft. There are a number of well document 120-170 mph Diamond Dusts, Magnums, Quickies and such. Look through the treads.
For reference, a full blown AMA 428 Q-500 (modern, composite) with a Jett/Nelson .40 engine typically flys in the 160-170 mph range.
The same Q-500 with a Jett SJ-50 will perform at about the same speeds - just has a more user friendly engine with a throttle.
Thats about the scope of things
Bob