If you are looking for an affordable 1 man tent the biggest issue you are going to come across is space, since you already know what tent size you want. You will have many options from different brands and in varying price ranges and sizes.  Most likely if you are solo camping you aren’t too concerned about having massive amounts of space and you wouldn’t want to look at cabin tents for just yourself to sleep in!  You are camping solo or backpacking or cycling for an extended period of time and just want something that is lightweight and will do a pretty good job at keeping you out of bad weather. 
However, space isn’t just about how many people can sleep in the tent.  It also is going to be about how confined of a space you are comfortable sleeping in and how tall you are.  A taller person is going to have a harder time finding a comfortable tent to sleep in, especially a one person tent. 
Your main options are bivy tents, dome tents and half dome tents.  The bivy tent is going to feel the most confined and only give you a really small covering that extends over your body.  Likely you won’t even be able to sit up in this tent.  The half dome will be smaller than a normal dome tent but will give you much more space than a bivy.  This might be a good option if you don’t need as much “breathing” room as a dome tent, but don’t want to be confined either.  Unlike the bivy you should be able to sit up.  A dome is going to give you the most area to sit up in since the tallest point will be in the center.  If you don’t want to feel too confined this would be the best option.  Obviously, the larger the tent the heavier it is going to be.

Cyclists might want to check out special bike tents that are designed specifically for you.  Generally, you can just use a regular tent but one way to save on size and weight is to go with a tent that allows you to forego tent poles and use your front tire instead to support the body of the structure.

Some Options Available for a 1 Person Tent:
The Eureka Solo Backcountry Tent gets great reviews by many backcountry campers.  It is a half dome, lightweight tent, weighing only 4 lbs, and allows some sitting room (The center is at about 3 ½ ft tall.  It is easy to set up, comes with a rain fly and yet, still provides good ventilation helping keep the condensation at a minimum.  However, the price tag might be higher than most hope for.  It’s list price sits at $170 but I’ve seen it at some places for $122.
Wenzel tents are usually very cheap tents and they are probably going to offer the cheapest priced tents you can find.  One option from Wenzel is the Wenzel Lone Tree Hiker Tent. This is a much more affordable one man tent and actually can fit up to 2 backpackers.  Trying to fit two campers in here with their gear however, would be a pretty tight fit.  Its list price starts at $35.99 but can be found for less than $30.
It comes with a rain fly with waterproof tape already applied.   The rain fly extends over more than just the top center of the tent .  This tent is a half dome style but the quality of this tent will not be real sufficient if you are more than a casual camper and plan to use it a lot. 

A more expensive tent option is the MSR Hubba Tent: 3 season.  MSR Hubba sells many tents at fairly hefty prices.  The list price on this particular one is $280 which is a a lot, but it could be worth it if you camp in the backcountry often.  It provides plenty of headroom when sitting upright and has a fully waterproof rain fly.  It is ultralight and easy to set up. It keeps condensation at a minimum, which could be a good option for nature photographers.  They also sale an optional footprint for this particular tent allowing you to set it up just with the rain fly and footprint.

Another trusted brand in camping gear is Coleman.  They offer a 1 person tent option called the Kraz x1 tent.  This is going to sale in the medium range as far as price range is concerned.  This is a hoop style tent, which means it’s kind of in between the half dome or bivy.  Instead of being a dome it is similar to the bivy in that it mainly fits around your body but it has rises a little more in the front giving you some extra space to sit up inside the tent.  This one is lightweight, includes a rain fly and is pretty breathable.    It can be found for around $80.