Lowest cost speaker: get the Lii 15" full range drivers and mount each on a plywood board. Connect your drivers directly to your speaker taps. Simplest and best sounding speaker for the money ($500 including the plywood.) Sound is holographic. No money wasted on shiny cabinet, additional drivers, crossover networks and speaker connectors. Want even more bass? Get a sub. You may not need it.
A controversial takes on low cost speakers with a lesson...
A controversial takes on 5 speakers comparison with an acoustic lesson few will catch: