I built a bench for my balcony for less than 30€ (including paint and screws). Note: i used really cheap wood. Just make sure that you that you predrill plus it means more sanding work.
6x 23mm 100mm 2m
1x 35mm 58mm 2.5m
1x 76mm 78mm 2m
Paint: moss green, 750ml; I painted twice