Let me point out that Intel HAD a monopoly on x86 CPUs until AMD's release of the Am386DX in 1991 (okay, there were the NEC V20/V30 clones, and some even more obscure ones, but no meaningful marketshare).
So we don't have to ask ourselves 'what would happen', we can just look at history and see what actually DID happen.
CPU prices never changed really. They are completely determined by what people are willing to pay for a CPU.
CPU development also never froze. Before AMD, Intel had already developed the 4004, 8008, 8080, 8086/8088, 80186, 80286, 80386 and 80486. Moore's law in full effect, with no competition (yes, that Moore in Moore's law is Gordon Moore, one of Intel's founders).
So can we please drop all the crackpot theories? Nothing bad happened when AMD wasn't around to compete. Know your history.
Likewise, we know that when AMD was around to compete, eg with the Athlon/Athlon64, the result was not Intel prices going down, but AMD prices going up... with AMD introducing their Athlon FX series at $1000+ prices and such.
Know your history.
My first three PCs were bought in the pre-AMD era (8088, 80386SX and 80486DX2), I've always paid pretty much the same price for a new PC, and always had pretty much a 'mainstream' performer.