It would be worth looking for a shop that deals with music equipment (e.g. amplified guitars, music recording eqipment, etc.) as some of those stores would have small PA amplifiers that would certainly not cost that much. Some electronics stores also carry PA amplifiers. This one (Irish shop priced at â‚¬86) gives an idea of what would work. The sound quality would not be as good as from a home theatre amplifier, but chances are that you'll unlikely notice any minor distortion or background hiss outdoors.
If you don't mind doing volume control, channel equalisation, etc. from your PC, a cheaper option would be to get a small in-car amplifier (e.g. 2-channel 100 watt), a 12 or 13.8 volt desktop power supply (preferrably with a 5+ amp rating) and a 3.5mm to 2x RCA audio cable. I was given 2 x 160 watt Hi-Fi speakers as a gift a while back and ran into the same issue, so I bought a 2nd hand in-car 440 watt amplifier for â‚¬50 and used a 13.8v 8amp CB transformer (typically sold for â‚¬40 at the time) and have been using this setup for the past 6 years. The only thing I actually miss, which a regular amplifier would have is a volume knob.