Those are decent wheels you have now. I believe that upgrading wheels for a "reasonable cost ??" might probably give marginal performance improvements. I would save my cash until I was able to stretch a bit more and get clearly superior wheels. You would probably realize more significant performance gains in the meantime if you tend to the engine: continue to improve overall health and fitness, and ride more.
Also if you are using heavy thick casing/sidewall tires you might try something lighter and more supple. They may ride better, and can roll faster--marginally, again, but at lower cost than a just-slightly-better wheel set.