I remembered for my 1st pregnancy, I bought my maternity pants/capris at Tom & Stefanie (house brand). Not very expensive. Good quality as well. I think around $29.90-39.90 per piece if I don't remember wrongly. Think got 15%-20% off discount as well. Until now, after been so many washes, my pants/capris still in good condition.
I think those branded maternity wear (Spring, Perfect Mum, Swell, Marton Bell etc) is very costly and expensive. But if u intend to have more kids (2-3), then can invest on it cuz can reuse later.