I was on a solo trip to Thailand quite recently. My trip included Bangkok as well. Since I was on a shoestring budget, I ensured that I spend as little as possible.
The entire trip cost me 30-32k. This includes the flight tickets (18k) and some islands such as phi phi and Krabi. I would suggest that you live in a hostel as they are cheaper and a fun way to meet new people (something a hotel doesn't offer). If Bangkok is the only place you plan on visiting, take the ferry to travel around the city as it is 9 to 10 times cheaper than other modes of transportation. Take a bus from the airport to khao San (60 baht) instead of the cab (300 baht). The train will take a lot of time but you can reach Khao San road (place where all the tourist live + cheap hostels) but it's the cheapest. DO NOT CONVERT YOUR CURRECY AT THE AIRPORT. Either get it converted from some agency in your home country or some agency in Bangkok. The conversion rate is twice so you get double the amount.