Triumph Bonneville T100 uses a 12 Volt, 10 Ah, YTX12-BS battery type. For the prices and availability, we would suggest you to please visit the nearest authorized service center.
Check the bike manual for specs , it is YTX12-BS OEM battery by YUASA , you can go for alternatives like VARTA , TATA GREEN (YUASA) if you don't get the OEM
As MrJaersj told it's very less..that's where triumph scores. sometimes nil...
Service the bike at regular intervals of 1yr or every 10K mileage cost will vary from 15K to 25K depending on wear and tear to replace parts like airfilter, disc pads, brake fluids, engine oil, gear box oil etc