Really liking what you're producing here - very cool tones to the background and yet with that nice punch of colour over the subject. The lighting is also very nice and soft.
Kmh summed up myfeelings about your second shot - the depth of field is too shallow - and I would have prefered to have had that protruding detailed section more in focus (either by changing your angle of shooting; your focus point and/or using a smaller aperture - though try not to go smaller than f13 if you can otherwise your shots will start to soften as a result of diffraction.)
As for a cheapy macro lens there are a few you can consider.
First off if you have an interest in flowers chances are that (at some point) you'll develop a liking for bugs as well - if that is the case then you want a macro lens of at least 90mm or longer in focal length. You can work with shorter but it is harder to master.
Lenses in that bracket are:
Tamron 90mm macro
Sigma 105mm macro
Tokina 100mm macro
Nikon 100mm VR macro
Sigma 150mm macro
Sigma 180mm macro
Nikon 200mm macro ? (I think there is one of those)
All of those lenses will give out about the same image quality/optical performance and whilst tests might show slight variations in the field you won't notice much of a working difference in that regard. What you will want to look at are the features of each lens such as:
Focusing setup and speed (eg HSM, USM focusing setups or not)
teleconverter compatability
Internal focusing
VR (vibration reduction).
From that line up the ideal lenses I would personaly go for would be the nikon 105mm VR because of its VR - a boon for non-macro work and a bit of aboon for macro (it has less of an effect, but less is better than none at all). After that there are the sigma 150mm and 180mm lenses to consider that offer a longer reach whilst also sporting great features. (eg internal focusing, teleconverter (sigma brand) compatabiltiy and HSM focusing).
The sigma 105mm, Tokina 100mm and Tamron 90mm have lesser features overall and are the cheaper options- but certainly their build and image quality are up to standard. The tamron is especaily popular as the budget macro lens for those with an interest in macro work.
As for shorter lenses there are a few:
Tokina 60mm macro
Nikon 60mm macro
sigma 70mm macro
Of the three the sigma and the nikon are fullframe compatable (the sigma also fits sigma teleconverters even those its not listed to do so) and would be my choices.
As for what makes a shot macro it depends: The strict rule is that a macro shot is where the image reflected onto the sensor by the lens is the same size as the subject is in real life. So if you have a 2mm insect its reflected onto the sensor as a 2mm sized image.
This is called "True macro" or 1:1 macro.
However 1:2 macro shots (half life size) are often considered as macro - such shots would be many flower, butterfly, dragonfly and such sized shots. Even though they are strictly not macro most people consider them to be so.
Going the other way you get 2:1, 3:1 etc.. where the subject is larger on the sensor than it is in real life. A harder (far harder) area to master working in. Currently only one canon lens on the market nativly goes into those higher magnifiactions with the rest all stopping at 1:1 - however there are many methods for getting more magnification if you get a desire to do so.