Honestly, if you're mic'd up anyhow, a small combo is plenty for almost anything.
I used to play my 5150 and a Laney 4x12 in my old church (all I had...). I wasn't mic'd, and it was in an old gymnasium, and it never went past 3 lol.
Yeah, it gets pricey. I haven't bought an amp in about 9 years.
My 5150 was $500 used. A decent 2x12 will run you about $300, and a good 4x12 will be at least $400-$600.
A 4x12 has a great tone, but I like it most for it ominous presence on stage bwahahahahaha... sorry. I'm cool now.
A head/cab setup gives your more flexibility with your tone since you can change cabs, etc, but really, a good combo will serve you for your entire life.