I got 1 decent pair of work trousers and a jacket to go with that to work as a suit, a mat shirt and about 4 plain mat tops (2 sleeveless and 2 long sleeved, at least 2 were white, 1 was black and I think 1 pink). I also got a pair of jeans and a couple of belly bands (latter from H&M).
I got the suit at 10 weeks, to make sure I had clothes suitable for work. I got the jeans the same day. The tops I picked up later on as the bump grew, and the bands. And I also got a (really useful!!) long black cardigan to go over things - I hadn't bothered to get a mat coat and the cardigan, with a few layers underneath and a winter scarf, worked sufficiently well even for a late December baby. I also got 1 other pair of work type trousers, which were dark grey pinstripe (so not nearly so useful - but worked with my jumpers/tops ok when I wasn't wearing a jacket of some sort).
I also had 3 normal M&S polo neck jumpers (2 black, 1 red) that I figured would stretch (and I was prepared to sacrifice as I had about 6 at the time - but they went back into shape afterwards surprisingly well). I made good use of scarves and long necklaces that I already had to make "black trousers and polo neck" look different. I wore the shirt as a jacket over the white tops somedays, and the shirt under the proper jacket on others. And I could also wear the black trousers with the black cardigan as a much more casual outfit (not office wear but for smart casual - like Christmas Day etc).
At home, I wore a pair of regular yoga pants a lot for comfort (they dipped below the bump) and large t-shirts. I had found an XXL rugby shirt (for the province I support anyway) on clearance, which I brought to an alterations place to get the sleeves taken up (not being the 6'4" and 18st man it was designed for! 
) and that was great for casual wear over vests, regular tops and the mat tops, with the jeans or yoga trousers, for less than the full price cost of a rugby jersey. And I had an already old pair of leggings that I slobbed around in with the cardigan hiding a lot.
The hardest thing was sorting bras, both while pg as my shape changed, and afterwards when I was bf'ing.
My suit was expensive (but necessary as I did work in a professional office), but I got a LOT of wear out of it. I also got a lot of wear out of the jeans and the shirt, so they were worth the maternity boutique price I paid for them. The rugby shirt was great to slob around in. The H&M mat range at the time was decent for plain tops at reasonable prices.
And don't forget that you don't immediately shrink back to pre-pg size and shape. I was surprised how much "bump" was still there for a couple of weeks after the birth, before that eased back in, quite apart from losing the extra pounds I had gained. So I was still wearing a number of those clothes for most of the spring as well - I probably used them about 9 months fairly constantly in the end.