Doing a solid, all over blonde may be cheaper, but it requires more maintenance which means you’ll be coloring your hair twice as often as you are now and it will probably come to the same price as you’re spending now. My recommendation for you is to keep doing highlights. Get fewer, piecey balayage highlights instead a lot of little highlights all over. You could even break up the ends with some lowlights to get some of your natural color back in there. This look will grow out nicely and naturally so you wont have to get it done so often.